US-Japan Strategic Synergy in Southeast Asia and Its Impacts

Xiang Haoyu is Special Research Fellow at the Department for Asia-Pacific Studies, China Institute of International Studies (CIIS); Bao Zhipeng is Assistant Research Fellow at the Department for Asia-Pacific Studies, CIIS.

With the United States accelerating the implementation of its Indo-Pacific strategy aimed at containing China, the USJapan alliance is transforming from a bilateral defensive pact to an outward-looking alliance with both offensive and defensive capabilities.Focusing on the Indo-Pacific region, the two countries promote synergy between their bilateral alliance and the broader US-led global network of alliances and partnerships, collective security arrangements, and regional cooperation platforms.Southeast Asia, strategically positioned at the confluence of two major oceans, has also witnessed increasing American and Japanese strategic engagement.The US and Japan’s diplomacy toward Southeast Asia tries to outcompete China.Through coordination and collaboration, their objective is to gain complementary advantages and achieve multiplier effects.At the same time, they are expanding their influence and presence within the ASEAN-led institutional network, and actively persuading individual ASEAN countries to participate in the Indo-Pacific framework they lead.Southeast Asia is emerging as a pivotal region for the growth of the US-Japan alliance,serving as a testing ground for the global expansion of this alliance.The actions taken by the US and Japan have significant implications for the success of their Indo-Pacific strategy.They are likely to exert competitive and divisive pressures on China-ASEAN relations.This article focuses on the US and Japan’s collaboration in Southeast Asia.By analyzing their coordination methods and cooperation mechanisms, it aims to unveil the development trends and influence space of US-Japanese relations with Southeast Asian countries and provide inputs for China’s diplomacy in Southeast Asia.

US-Japan Strategic Synergy in Southeast Asia

Since the Biden administration took office, the United States has extended its outreach and partnership with Japan, with the latter’s foreign strategy aligning more closely with that of the United States.The US-Japan alliance has grown stronger, and increasingly emphasized its strategic focus on vying for dominance in the Indo-Pacific regional order.Southeast Asia is strategically positioned at the crossroads of the Pacific and Indian Oceans.As early as the Obama administration launched the strategy of rebalance to the Asia-Pacific, there were discussions and policy suggestions in the US and Japanese strategic communities advocating joint expansion in Southeast Asia.Now, Southeast Asia has increasingly become the main focus of the two countries’ Indo-Pacific strategy, and official dialogue mechanisms in this regard have been launched.With the growing US-Japan strategic synergy in the Indo-Pacific region, several new features have emerged in their collaborative expansion efforts in Southeast Asia.

Converging strategic goals

After World War II, the United States leaned on the Asia-Pacific alliance system, including the US-Japan alliance, to sustain its long-term hegemony in the Western Pacific.The core interests of the US and Japan align in their collective efforts to uphold the regional political, economic, and security framework they lead.As apparent from their respective Indo-Pacific strategies, these two countries share three primary objectives in jointly promoting their diplomacy in Southeast Asia: preserving their geopolitical influence, broadening their economic interests,and competing with China to contain its rise.A collaborative US-Japan report released in 2016, “US-Japan Relations and Southeast Asia: Meeting Regional Demands,” identified five drivers for US-Japanese ties with Southeast Asia:common commercial interest; Southeast Asia’s geopolitical value; increased regional production and trade integration among North America, Northeast Asia, and Southeast Asia under the multilateral architecture; Southeast Asia’s position as a key extension for the US and Japan to increase their bilateral cooperation; and the need to deal with China’s rising assertiveness.1The East-West Center and the Sasagawa Peace Foundation, “US-Japan Relations and Southeast Asia:Meeting Regional Demands,” 2016, https://www.spf.org/en/global-data/user19/US-Japan_and_SEA.pdf.In the joint statements issued after the US-Japan summits in 2021 and 2022, the two countries mentioned they “support ASEAN’s unity and centrality in the Indo-Pacific, as well as the ASEAN Outlook on the Indo-Pacific.”2“US-Japan Joint Leaders’ Statement: US-Japan Global Partnership for a New Era,” Ministry of Foreign Affairs of Japan, April 16, 2021, https://www.mofa.go.jp/mofaj/files/100202832.pdf; “Japan-US Joint Leaders’ Statement: Strengthening the Free and Open International Order,” Ministry of Foreign Affairs of Japan, May 23, 2022, https://www.mofa.go.jp/mofaj/files/100347254.pdf.This demonstrated their unanimous recognition of the importance of ASEAN.

From the series of policy statements and actions by the US and Japan in Southeast Asia, their collaborative expansion in the region appears to have three strategic objectives: first, to open a new platform for third-party cooperation, and collectively expand into Southeast Asian markets to further their respective economic interests; second, to bolster their competition against China, positioning Southeast Asia as a key battleground for exerting influence and rivalry; third, to forge a new pivot to reshape regional order,thereby redefining the dominant position of the US and Japan.Given the United States’ categorization of China as its biggest strategic rival and China’s growing influence within ASEAN, the three objectives mentioned above can be consolidated into a single overarching goal of outcompeting China.3The Biden administration’s National Security Strategy, released in October 2022, repeatedly mentions the term “outcompete” and takes “outmaneuvering China in geopolitical competition” as the US national strategic goal.As China has significant influence in Southeast Asia, completely excluding Chinese influence is no longer realistic.Promoting diplomacy in Southeast Asia through complementary advantages is a more logical choice.

Complementary resources and tools

Looking at the diplomatic layouts of the US and Japan, Japan attaches greater importance to Southeast Asia due to geographical proximity and significantly closer economic connections.Thus, Japan has devoted substantially more resources to the region.In contrast, the US tends to place Southeast Asia as part of its Asia-Pacific layout, with limited overall attention and strategic input.Even though the US Indo-Pacific strategy has elevated the status of ASEAN in recent years, its effectiveness is stll constrained by inconsistent policy and insufficient real investment.Comparing the respective influence of the US and Japan on ASEAN, the US, as the main provider of security assets for Southeast Asian countries, especially maritime ASEAN states, has a greater influence on regional security affairs, while Japan has a relatively prominent influence in economic, social, and livelihood areas given its long-term official development assistance and investment footprint in the region.The differences between the two’s relations with Southeast Asian countries create certain complementary advantages and drive their joint expansion in the region.

First, regarding the cooperation mechanism with ASEAN, the US can leverage Japan’s relatively complete and mature platforms in the region.Japan has had a “10+1” mechanism with ASEAN for many years, with close exchanges and cooperation at all levels and in various fields, including summit meetings.Additionally, early in 1981, Japan had established the ASEANJapan Center to promote trade, investment and tourism, and in 2006, it launched the Japan-ASEAN Integration Fund (JAIF) to support the building of the ASEAN Community.4Japan has so far provided $860 million to ASEAN countries through the JAIF, which has implemented more than 560 projects.See “Japan-ASEAN Integration Fund (JAIF),” Ministry of Foreign Affairs of Japan,April 13, 2023, https://www.mofa.go.jp/mofaj/area/asean/j_asean/jaif.html.In 2008, the two sides signed the Agreement on Comprehensive Economic Partnership.In contrast, the US has had relatively few institutional arrangements specifically for ASEAN.Despite a US-ASEAN summit mechanism and a diplomatic delegation in ASEAN, it lacks a permanent and institutional cooperation platform and special funding support,and it has not yet signed a free trade agreement with ASEAN.

Second, regarding security, Japan must borrow the US influence to expand its presence in Southeast Asia.The US maintains military presence in the region through its Southeast Asian allies and partners such as the Philippines, Thailand and Singapore, and it provides security public goods under the pretext of safeguarding important shipping lanes.The US presence has exerted a strong security influence on maritime ASEAN countries.Against the background of China’s comprehensive and rapid rise, ASEAN needs the US to counterbalance China’s geopolitical influence in its strategy of managing relationships with major powers.Bilahari Kausikan, former Permanent Secretary of Singapore’s Ministry of Foreign Affairs, stated that in the context of the US-China confrontation, ASEAN countries tend to take opportunistic actions, and the US military presence is indispensable for regional stability.5Bilahari Kausikan, “US Military Presence as the Foundation of Diplomacy toward China,” Diplomacy(Japan), No.76, 2022.Japan’s strengthened security cooperation with Southeast Asia, including its involvement in hotspot issues such as the South China Sea, relies mainly on the existing security architecture and military presence of the United States.In non-traditional security, the US and Japan’s cooperation with Southeast Asia is complementary.For example, Japan has been involved in piracy prevention in the Malacca Strait since the late 20th century through various channels and at multiple levels.It has established a close cooperation mechanism with Indonesia, Malaysia and Singapore, the three countries in the Malacca Strait area.The earlier Japanese presence has provided a new platform for the US and Japan to strengthen their regional security influence.Additionally, in cybersecurity, the US has provided training materials to the ASEAN-Japan Cybersecurity Capacity Building Centre, which Japan has given funding support.This constitutes a typical case of complementary cooperation between the two countries.

Third, the development of relations with ASEAN countries differs between the US and Japan, with the US relying on Japan to a certain extent in its dealings with ASEAN.Japan’s relations with ASEAN countries are relatively well-balanced, and Southeast Asian people in general have a favorable view of Japan.In comparison, the US relations with ASEAN countries present a hierarchical feature with varying degrees of closeness.The first level is treaty allies, including the Philippines and Thailand, along with security partners like Singapore; the second level comprises countries that the US focuses on and pays special attention to, including Indonesia, Vietnam and Malaysia; and the third level includes nations of lesser concern or those with relatively distant relations, including Cambodia, Laos, Brunei and Myanmar.In recent years,the US has intentionally leveraged Japan’s political connections, economic relations, and the social and human ties forged through Japan’s long-term,extensive engagement in Southeast Asia to compensate for its limited influence in those areas.For example, in 2022, when American journalist Danny Fenster was arrested in Myanmar, Japan helped the US mediate through its connections and prompted the Myanmar military to release the journalist.6“US Government Sought Help of Sasakawa in Fenster Release,” Mizzima, November 19, 2021, https://mizzima.com/article/us-government-sought-help-sasakawa-fenster-release.

Fourth, the US and Japan have different official aid and investment priorities in Southeast Asia.Southeast Asia is host to a bunch of emerging economies and developing nations with substantial development potential,making it a pivotal region for both the US and Japan’s official development assistance (ODA) and foreign direct investment (FDI).In terms of foreign assistance, Southeast Asia has been a major destination for Japan’s ODA since the 1960s.In 2021, five Southeast Asian countries ranked in the top ten recipients of Japan’s ODA,7The Southeast Asian countries that rank among the top ten recipients of Japan’s ODA are the Philippines,Indonesia, Cambodia, Vietnam, and Myanmar, ranking the 3rd, 4th, 5th, 7th, and 8th respectively.with more than $3.6 billion in aid.8“ODA Performance in OECD/DAC,” Ministry of Foreign Affairs of Japan, April 19, 2023, https://www.mofa.go.jp/mofaj/gaiko/oda/shiryo/jisseki.html.In comparison,the US aid to Southeast Asia is much smaller, with the Philippines, Vietnam,Indonesia, Cambodia, and Laos ranking the top five for US aid to the region in 2021, totalling only $480 million.9“Aid (ODA) Disbursements to Countries and Regions,” https://stats.oecd.org/Index.aspx?DataSetCode=TABLE2A.Specifically, Japan’s ODA focuses on social and livelihood areas with a strong emphasis on the grassroots and capacity building, while the American ODA has a stronger military and ideological focus, which, apart from humanitarian assistance, tends to be used for the promotion of “democracy” and “good governance.” In terms of investment,the US accumulated direct investment in ASEAN had reached $329 billion as of 2019, higher than its investment in China, Japan, South Korea and India combined.In 2021, the US direct investment in Southeast Asia reached$40 billion.10“ASEAN Stats Data Portal,” July 31, 2023, https://data.aseanstats.org/fdi-by-hosts-and-sources.The US investment in Southeast Asia is characterized by the dominance of multinational corporations and a high proportion of nonmanufacturing sectors such as finance and insurance.In 2021, Japanese direct investment in ASEAN countries increased by 58 percent, reaching 3.1 trillion yen and accounting for 19.1 percent of Japan’s total foreign direct investment.11“In 2021, Japanese Direct Investment in ASEAN Countries Increases by 58%, Reaching 3.1 Trillion Yen,” Japan External Trade Organization, February 10, 2022, https://www.jetro.go.jp/biznews/2022/02/59fd96f21af8e4fc.html.The Japanese investment in Southeast Asia is concentrated in manufacturing,including automobiles, electronics, and textiles.

Synergy in implementing measures

Driven by the common goal of advancing the Indo-Pacific strategy, the US-Japan diplomacy toward Southeast Asia is shifting from uncoordinated individual efforts to well-communicated and highly coordinated cooperation.This process has demonstrated the characteristics of strategic guidance, policy synergy, resource sharing, and coordinated action from the top down.

At the strategic level, the US and Japan incorporate the cooperation with ASEAN into the Indo-Pacific framework and align it with the ASEAN Outlook on the Indo-Pacific to expand their strategic convergence with ASEAN.The declassified version of the Indo-Pacific Strategic Framework released by the Trump administration in January 2021 clearly stated that ASEAN centrality is key to achieving the goal of a “Free and Open Indo-Pacific.” The US should promote and reinforce Southeast Asia and ASEAN’s central role in the region’s security architecture, deepen its relationship with Singapore, Malaysia, Vietnam, and Indonesia, and encourage Japan to strengthen its cooperation with Southeast Asian countries.12“US Strategic Framework for the Indo-Pacific,” January, 2021, p.9, https://sgp.fas.org/news/2021/01/indopac-framework.pdf.In its Indo-Pacific Strategy of the United States released in February 2022, the Biden administration reaffirmed the US endorsement of ASEAN centrality, and vowed to deepen long-standing cooperation with ASEAN, which was listed as the fourth of its ten priorities.Since Prime Minister Shinzo Abe put forward the concept of a “Free and Open Indo-Pacific” during his term of office,Japan has placed ASEAN in a key position to connect the two continents of Asia and Africa as well as the Indian and Pacific Oceans, respecting ASEAN centrality and unity and proposing to enhance connectivity between ASEAN countries, promote “quality infrastructure,” trade and investment, and improve the business environment and human resources in the region.13“Free and Open Indo-Pacific,” Ministry of Foreign Affairs of Japan, June 10, 2023, https://www.mofa.go.jp/mofaj/files/000430631.pdf.In March 2023, Prime Minister Fumio Kishida launched Japan’s New Plan for a “Free and Open Indo-Pacific (FOIP),” proposing the expansion of the four pillars of FOIP cooperation, with a focus on cooperation with ASEAN.It also reaffirmed that Japan and ASEAN will promote practical cooperation based on the action plan for the ASEAN Outlook on the Indo-Pacific and guided by the joint statement of their summit in 2020.14“New Plan for a ‘Free and Open Indo-Pacific (FOIP),’” Ministry of Foreign Affairs of Japan, June 10,2023, https://www.mofa.go.jp/mofaj/files/100477659.pdf.

At the policy level, the US and Japan have strengthened bilateral external cooperation through summit meetings, foreign and defense ministers’ “2+2”consultations, and economic “2+2” cooperation to build consensus and clarify the guidelines for cooperation in various fields.The US-Japan joint leaders’statements and the document for the US-Japan Competitiveness and Resilience(CoRe) Partnership specify the focus areas and objectives of US-Japan cooperation in the new era, forming a policy guidance for the two countries to jointly expand their presence in Southeast Asia.The US and Japan have also strengthened their special policy coordination through the Southeast Asia Strategic Dialogue mechanism launched in June 2022.

At the operational level, the US and Japan have employed a range of multilateral initiatives and cooperative dialogue platforms in which both countries participate.They use various instruments including FDI and ODA to mutually reinforce and broaden their political and security influence in Southeast Asia while enhancing cooperation with regional countries.The main mechanisms and platforms include regional multilateral arrangements such as the US-Japan-Australia-India quadrilateral mechanism (Quad) and the Indo-Pacific Economic Framework (IPEF) led by the US and Japan, as well as the East Asia Summit (EAS), the ASEAN Regional Forum (ARF), the ASEAN Defense Ministers’ Meeting Plus (ADMM+) and other ASEAN-led mechanisms.

US-Japan Synergy on Indo-Pacific Strategy

In their joint engagement in Southeast Asian affairs, the United States and Japan have emphasized the alignment of their efforts with ASEAN within the Indo-Pacific framework.Their moves, ranging from mechanism arrangements to functional cooperation, have all demonstrated their consideration to integrate ASEAN into their broader Indo-Pacific strategy.

Mechanism arrangements

The US and Japan’s joint efforts in Southeast Asia primarily depend on the Indo-Pacific cooperation platforms they lead and the US architecture of alliances in the Asia-Pacific.The two pillars are reflected in three levels of institutional arrangements.

The first is cooperation with Southeast Asia within the Indo-Pacific strategic framework.The US and Japan rely on the Quad mechanism and the IPEF to cooperate and interact with Southeast Asian countries on maritime security, economy and trade, supply chains, infrastructure and other issues.

The Quad is a key channel for the US and Japan to coordinate the promotion of the Indo-Pacific strategy and an important platform for the two countries to jointly advance cooperation with Southeast Asia.The accelerated implementation of their Indo-Pacific strategy, while making the Quad mechanism grow in depth and substance, has sparked concerns among Southeast Asian countries, who are worried that an active Quad could accelerate great-power competition in the region and undermine ASEAN centrality.15“Southeast Asians’ View of Quad and AUKUS: Some Thaw, But Not Yet Warm,” Fulcrum, February 23, 2023, https://fulcrum.sg/southeast-asians-view-of-Quad-and-aukus-some-thaw-but-not-yet-warm.After the outbreak of the COVID-19 pandemic, vaccine assistance provided an opportunity for the US and Japan to expand cooperation with ASEAN countries and reduce their worries about the Quad.The March 2021 Quad summit announced the Quad Vaccine Partnership that saw 1 billion doses of COVID-19 vaccines provided to Southeast Asian countries, with the US and Japan responsible for the most critical vaccine production.16“The US, Japan, Australia and India in the Quad Meeting with Different Calculations,” Lianhe Zaobao,March 16, 2021, https://www.zaobao.com.sg/wencui/politic/story20210316-1131674.The ASEAN countries have since increasingly considered Quad as a platform to keep the US committed to the region.17“The Quad and ASEAN — Where to Next?” East Asia Forum, June 25, 2022, https://www.eastasiaforum.org/2022/06/25/the-Quad-and-asean-where-to-next.

Starting from vaccine production and supply, the US and Japan have continuously expanded their functional cooperation within the Quad framework, actively engaging with Southeast Asian countries in areas such as humanitarian assistance and disaster relief (HADR) and climate change.In March 2021, the US, Japan, India, and Australia announced the establishment of the Quad Critical and Emerging Technology Working Group.In May of the same year, Japan provided disaster relief technical training and equipment worth about $1.1 million to the Philippine military.The Quad’s efforts in the above areas have gradually earned positive responses from Southeast Asian countries.According to “The State of Southeast Asia: 2022 Survey Report” conducted by the ASEAN Studies Centre at ISEAS-Yusof Ishak Institute in Singapore, 58.5 percent of respondents support the strengthening of cooperation between Quad and Southeast Asian countries in areas such as vaccine safety and climate change.18ISEAS-Yusof Ishak Institute, “The State of Southeast Asia: 2022 Survey Report,” February 2022,https://www.iseas.edu.sg/wp-content/uploads/2022/02/The-State-of-SEA-2022_FA_Digital_FINAL.pdf.The joint vision statement released at the ASEAN-US special summit in May 2022 also welcomed support extended by ASEAN dialogue partners through initiatives like the Quad Vaccine Partnership.19“Joint Vision Statement of ASEAN-US Special Summit 2022,” The White House, May 13, 2022,https://www.whitehouse.gov/briefing-room/statements-releases/2022/05/13/asean-u-s-special-summit-2022-joint-vision-statement.Moreover, the Quad has expanded cooperation with ASEAN to maritime security.In May 2022, the four Quad countries officially launched the Indo-Pacific Partnership for Maritime Domain Awareness (IPMDA)initiative in Tokyo, which specifies that Quad countries would contribute to the region’s maritime domain awareness over the next five years by combating illegal fishing, and improve regional countries’ ability to monitor maritime activities such as “dark shipping,” address extreme climate and humanitarian events, and protect their fisheries.It also mentioned strengthening support for the construction of the Information Fusion Center in Singapore.20“Fact Sheet: Quad Leaders’ Tokyo Summit 2022,” The White House, May 23, 2022, https://www.whitehouse.gov/briefing-room/statements-releases/2022/05/23/fact-sheet-Quad-leaders-tokyosummit-2022.

In the economic field, the US, with the active assistance of Japan, has launched the Indo-Pacific Economic Framework in an attempt to address its economic shortcomings in Southeast Asia.The US strategy in Southeast Asia has always had the problem of over-emphasizing security and downplaying economy.21“Lack of an Economic Strategy Is Hurting US in Southeast Asia,” Nikkei Asia, August 5, 2021, https://asia.nikkei.com/Opinion/Lack-of-an-economic-strategy-is-hurting-US-in-Southeast-Asia.Though the Biden presidency has put the US back to supporting multilateralism, it is difficult for the country to return to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) due to strong domestic opposition.To make up for this shortcoming, the Biden administration plans to introduce a new regional economic cooperation framework and regards ASEAN countries as important targets to win over,with Japan playing a key role in this effort.When the IPEF was first proposed,due to doubts about the US strategic intentions and ability to deliver on its commitments, Malaysia, Vietnam and other Southeast Asian countries took a wait-and-see attitude toward joining the IPEF.22“Given Their Economic Ties with China, Many Southeast Asian Countries Are Hesitant to Join the IPEF,” Lianhe Zaobao, May 17, 2022, https://www.zaobao.com.sg/news/sea/story20220517-1273370.To encourage ASEAN countries’ participation, Japan played an active mediating role and convinced the US to downplay the tone of countering China and lower the entry threshold while promoting the accession of ASEAN countries and India.23“What Does the IPEF Aim For?” Nikkei, February 6, 2023, https://www.nikkei.com/article/DGKKZO68215320W3A200C2EAC000.In May 2022, the US launched the IPEF with four pillars: quality infrastructure,resilient supply chains, clean energy, and fair economy.Seven ASEAN countries eventually joined the framework and participated in negotiations on all four pillars.

The second is the “US-Japan plus” pattern of minilateral cooperation.Based on the US-Japan alliance, the Biden administration has accelerated the transformation of the US alliance and partnership system in the Asia-Pacific into a more networked and layered architecture.Southeast Asia becomes the focus for Japan and the US to expand their minilateral network.

The US and Japan have actively participated in Mekong sub-region cooperation, coordinating and promoting in-depth functional cooperation with Mekong countries.In 2019, the two countries launched the Japan-USMekong Power Partnership (JUMPP) with the Mekong countries.Since Biden took office, the US and Japan have held four meetings with relevant parties on promoting Mekong countries’ power sector development, renewable energy deployment, and regional power trade mechanism construction.According to US Vice President Kamala Harris in November 2022, since announcing the JUMPP, the US and Japan have expanded partnerships with Mekong countries to build more competitive and connected energy markets and infrastructure.The Biden administration intends to request from Congress up to $20 million in additional funding to the JUMPP, which will support the Mekong subregion’s pursuit of energy security while encouraging greater regional power flows, clean energy integration, decarbonization, and resilience.24“Fact Sheet: Vice President Harris Announces New US Support for Clean Energy in the Mekong Region,” The White House, November 19, 2022, https://www.whitehouse.gov/briefing-room/statementsreleases/2022/11/19/fact-sheet-vice-president-harris-announces-new-u-s-support-for-clean-energy-in-themekong-region.In April 2023,the US and Japan released the JUMPP Action Plan, according to which the two countries will provide technical assistance and capacity-building support to the Mekong countries in the areas of floating solar generation, battery energy storage systems, and women’s engagement through appropriate training programs and roundtable joint research around the three pillars of clean energy integration, market development and investment, and regional power trade.25“Japan-US-Mekong Power Partnership (JUMPP) Action Plan,” April 2023, https://mekonguspartnership.org/wp-content/uploads/2023/04/JUMPP-Action-Plan-Final-April-2023-Web-3.pdf.

With the renewal and reinvigoration of the US-Philippines alliance and Japan’s new involvement, the US-Japan-Philippines trilateral security cooperation has gradually taken shape.Since taking office, Philippine President Bongbong Marcos has interacted frequently with the US and Japan.In May 2022, the US and the Philippines signed a maritime security framework agreement to enhance the interoperability of their maritime forces.At the inaugural US-Japan-Philippines Trilateral Defense Policy Dialogue (TDPD)in September of the same year, the three parties identified five priority areas of cooperation: maritime security, maritime domain awareness, cyber security,information sharing, and HADR.A regular dialogue mechanism was also established.26“Japan-Philippines-US Trialteral Defense Policy Dialogue (TDPD) Press Release,” Ministry of Defense of Japan, September 15, 2022, https://www.mod.go.jp/en/images/20220915a.pdf.In December of the same year, the jet fighters of Japanese Air Self-Defense Force landed at the Philippines’ Clark Air Base for the first time since World War II.During Marcos’ visit to Japan in February 2023, Japan and the Philippines agreed to sign a reciprocal access agreement as soon as possible.In addition, the US, Japan, and the Philippines also plan to establish a ministeriallevel trilateral dialogue framework to institutionalize their cooperation.27“Japan, Philippines and US to Set up Three-Way Security Framework,” The Japan Times, March 28,2023, https://www.japantimes.co.jp/news/2023/03/28/national/philippines-japan-us-security-framework.The US-Philippines joint statement issued during Marcos’ visit to the US in May 2023 proposed to establish US-Japan-Philippines and US-Australia-Philippines trilateral cooperation.28“Joint Statement of the Leaders of the United States and the Philippines,” The White House, May 1, 2023, https://www.whitehouse.gov/briefing-room/statements-releases/2023/05/01/joint-statement-of-theleaders-of-the-united-states-and-the-philippines/.In addition to building trilateral cooperation frameworks, the US, Japan and the Philippines are taking the opportunity of the resumed joint patrols in the South China Sea to motivate Australia’s participation and expand the trilateral cooperation to a quadrilateral one.

The third is the US and Japan’s cooperation with third-country partners in Southeast Asia.The US and Japan are using the “US-Japan +1” cooperation model as an important tool to deepen their alliance and jointly expand their influence in Southeast Asia.In the field of military security, the US encourages Japan to strengthen its defense and security cooperation with Southeast Asia and to replicate and expand the US-Japan “2+2” mechanism to Southeast Asia,thus complementing the existing US bilateral security cooperation frameworks in the region.After establishing a “2+2” consultation mechanism with Indonesia, Japan held the first foreign and defense ministers’ “2+2” meeting with the Philippines in April 2022.After a seven-year hiatus, in April 2023,the US and the Philippines also resumed their “2+2” dialogue mechanism.In terms of specific cooperation, with the increasingly offensive nature of the USJapan alliance, the space for defense cooperation between Japan and Southeast Asian countries has been further opened up.Particularly, Japan is seeking to transfer its defense equipment and technology to Southeast Asia.From 2021 to 2022, Japan successfully entered into agreements on transferring defense equipment and technology with Indonesia, Vietnam, and Thailand.The move expanded the number of regional countries with which Japan has such agreements to five.The joint statement of the US-Japan “2+2” meeting in January 2023 reiterated that both countries intend to further promote security cooperation with Southeast Asian countries through such activities as joint training, capacity building, and potential transfer of defense equipment.29“Joint Statement of the Security Consultative Committee (2+2),” US Embassy and Consulates in Japan,January 11, 2023, https://jp.usembassy.gov/joint-statement-security-consultative-committee-2plus2.In non-traditional security, the US and Japan have deepened cooperation with Southeast Asian countries based on the concept of “human security” proposed by Japan.The concept played a role in combating human trafficking in Thailand and promoting the Mindanao peace process and typhoon disaster relief in the Philippines.30Tomohito Shinoda, The Japan-US Alliance and Southeast Asia: Beyond Traditional Security, Chikura Shobo, May 2018.

In the economic sphere, amid the successive impacts of the COVID-19 pandemic, the Ukraine crisis, and the US acceleration of “decoupling” from China on global industrial and supply chains, the US and Japan launched the Economic Policy Consultative Committee, or the economic “2+2,” in July 2022, with a focus on transferring key supply chains to Southeast Asia.Based on the US-Japan CoRe Partnership and through mechanisms such as the Clean Energy Partnership, the Clean Energy and Energy Security Initiative, and the Japan-US-Mekong Power Partnership, the two countries have been cooperating on a series of projects that address the needs of Southeast Asian countries such as Vietnam and Indonesia in the areas of economic restructuring and industrial upgrading.

Key areas of cooperation

In specific areas of cooperation, the US and Japan have been jointly expanding their presence in Southeast Asia by fitting into ASEAN’s policy direction while leveraging their own strengths.Their cooperation layouts can generally be categorized into the following two paths.

The first is cooperation in the four priority areas of the ASEAN Outlook on the Indo-Pacific.In addition to reinforcing the mechanisms they lead, the US and Japan are accelerating the integration of the “Free and Open Indo-Pacific” concept with the ASEAN Outlook on the Indo-Pacific.Firstly, the US and Japan are actively deepening and enhancing their relations with ASEAN.Japan plans to establish a comprehensive strategic partnership with ASEAN to commemorate the 50th anniversary of their friendship in December 2023.31“Japan, ASEAN to Upgrade Ties in Dec.Amid Growing China, US Clout,” Kyodo News, March 7,2023, http://english.kyodonews.net/news/2023/03/7e259ed12b8b-japan-asean-to-upgrade-ties-in-dec-amidgrowing-china-us-clout.html.The US and Japan regard functional cooperation with ASEAN as an opportunity to strengthen coordination with regional allies and partners and promote the implementation of their Indo-Pacific strategy.They have actively strengthened policy communication with ASEAN countries through ASEANled platforms such as EAS, ARF, and ADMM+.During the ASEAN summit and related meetings hosted by Cambodia in November 2022, the US and Japan held intensive bilateral and trilateral meetings with South Korea and other regional allies and partners.The US, Japan, and South Korea jointly issued a statement, emphasizing that they will work together to strengthen deterrence in the Indo-Pacific region, uphold “the freedom of navigation and overflight,” and commit to strengthening cooperation with regional partners to support the implementation of the ASEAN Outlook on the Indo-Pacific.32“Phnom Penh Statement on US-Japan-Republic of Korea Trilateral Partnership for the Indo-Pacific,” The White House, November 13, 2022, https://www.whitehouse.gov/briefing-room/statementsreleases/2022/11/13/phnom-penh-statement-on-trilateral-partnership-for-the-indo-pacific.

Secondly, the US and Japan are actively seeking to integrate into the ASEAN institutional platform and participate in functional cooperation for the building of the ASEAN Community.The two countries have formulated corresponding cooperation plans with ASEAN around the four pillars of the ASEAN Outlook on the Indo-Pacific: maritime cooperation, connectivity, the Sustainable Development Goals (SDGs), and economic and other possible areas of cooperation.They support the ASEAN Community Vision 2025 goals in narrowing the development gap, enhancing connectivity, smart cities, and trade facilitation.In the context of the establishment of their comprehensive strategic partnership, the US and ASEAN have further promoted the Plan of Action to Implement the ASEAN-US Strategic Partnership (2021-2025)through regular mechanisms such as the Joint Cooperation Committee.Currently, Japan and ASEAN are ahead of the US in terms of cooperation on the ASEAN Outlook on the Indo-Pacific, with 89 projects in progress covering a wide range of areas such as connectivity, marine environmental governance, and people-to-people exchanges.The New Plan for a “Free and Open Indo-Pacific (FOIP)” announced by Prime Minister Kishida in March 2023 proposed continuous synergy with the priority areas set out in the ASEAN Outlook on the Indo-Pacific and promoting cooperation with ASEAN countries in the spirit of co-creation.

The second is security cooperation.The US and Japan rely on maritime ASEAN countries such as the Philippines, Vietnam, Indonesia, Malaysia, and Singapore to promote multi-level and multi-dimensional maritime security cooperation with a focus on non-traditional security.In terms of specific actions, they take the South China Sea dispute and non-traditional security issues as the starting point and work together to improve the interoperability of military forces with Southeast Asian countries.With the 2015 US-Japan Defense Cooperation Guidelines lifting the restrictions on the geographical coverage of Japanese Self-Defense Forces’ operations and officially proposing for the first time to “safeguard the navigation security in the South China Sea,”the two countries launched joint patrols in the South China Sea in 2016.Since the Biden administration took office, the US and Japan have intensified their military intervention in the area.In 2021 and 2022 respectively, the US and allied countries outside the region conducted 75 and 69 military drills in the South China Sea and surrounding areas, with Japan joining 61 of the drills in both years and ranking first among the extra-regional countries.33“2021 Report on Incomplete US Military Activities in the South China Sea,” SCSPI, March 2022,p.21, http://www.scspi.org/sites/default/files/reports/2021nian_mei_jun_nan_hai_jun_shi_xing_dong_bu_wan_quan_bao_gao_.pdf; “2022 Report on Incomplete US Military Activities in the South China Sea,”SCSPI, March 2023, http://www.scspi.org/sites/default/files/reports/2022nian_mei_jun_nan_hai_jun_shi_xing_dong_bu_wan_quan_bao_gao_.pdf.In November 2021, the ANNUALEX 2021, led by Japan and featuring US and Japanese maritime forces, was held in the Philippine Sea.The US Navy’s Nimitz-class aircraft carrier USSCarl Vinsonand Japan’s Izumo-class helicopter destroyer were among the capital warships that participated in the military exercise,during which the US 7th Fleet and the Japan Maritime Self-Defense Force held their first joint anti-submarine warfare operation.34“Australia, Canada, Germany, Japan and the US Join for Annualex 2021,” November 21, 2021, https://www.c7f.navy.mil/Media/News/Display/Article/2849963/australia-canada-germany-japan-and-the-us-joinfor-annualex-2021.

In non-traditional security, the US and Japan have cooperated in combating piracy and illegal fishing as an important measure to strengthen relations with ASEAN.Leveraging its role in the security mechanisms of the Strait of Malacca such as the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP), Japan has been exerting influence on maritime ASEAN countries through information sharing, joint research and capacity building.In the future, Japan may bring the US into play and coopeate with the US to strengthen control over the key shipping lanes in the region.

Limitations of the US-Japan Synergy

As the United States and Japan increase their diplomatic coordination in Southeast Asia, their joint expansion, characterized earlier by informality and spontaneity, are becoming more coordinated and evolving into more structured and synchronized efforts.In the future, they plan to reinforce their strategic guidance, policy alignment, resource sharing, and coordinate their actions in a top-down approach, seeking to achieve the goal of outcompeting China.However, it should be noted that the US-Japan collaborative efforts to expand in Southeast Asia are facing numerous challenges, including the divergent interests between the US, Japan and ASEAN, the different perspectives on regional order between the US and Japan on one side and ASEAN on the other, and the risk of security dilemma and fragmentation in the region caused by the US and Japan’s exclusive and often confrontational policies.All these will affect the effectiveness of their joint expansion.

Strategic shortcomings and insufficient investment

Investment from the US is the key factor in the effectiveness of US-Japan joint expansion in Southeast Asia.However, the US is currently constrained by its domestic political impasse and pinned down by other global hotspot events,making its substantive investment in ASEAN rather limited.Although the US has increased the emphasis on Southeast Asia in its Indo-Pacific strategy,it still lacks focused strategic planning and an all-around layout for the region.Some Japanese scholars have pointed out that the Biden administration has sent confusing policy signals toward Southeast Asia since taking office,with contradictory attitudes in many aspects such as values, attitudes toward China, and multilateralism.35Tomotaka Shoji, “Southeast Asia and the United States under the Biden Administration—From Hope to Confusion,” Sasakawa Peace Foundation, July 28, 2022, https://www.spf.org/iina/articles/shoji_19.html.In May 2022, the US invited leaders from the ten ASEAN countries to Washington for a special summit, but announced a measly $150 million investment in ASEAN to strengthen Southeast Asia in fields such as clean energy, maritime security and the fight against COVID-19,which disappointed ASEAN countries.36“The United States Promises to Invest 150 Million Dollars, But ASEAN Will Not ‘Take Sides,’” May 13, 2022, https://www.guancha.cn/internation/2022_05_13_639594.shtml.

At the implementation level, the US and Japan still lack strategic coordination on their investment in Southeast Asia.According to Kei Koga,neither the US nor Japan has a clear vision for ASEAN’s strategic role in the Indo-Pacific.Despite their supportive diplomatic posture toward ASEAN,there is no clear set of coordinated strategies between the two countries to empower ASEAN effectively.37Kei Koga, “Recalibrating US-Japan Indo-Pacific Strategies Towards ASEAN,” October 2022, https://www.wilsoncenter.org/sites/default/files/media/uploads/documents/2022-10_Recalibrating-ASEAN_Koga.pdf.In fact, the cooperation projects they have launched in Southeast Asia are mostly non-substantive, focusing on personnel training, capacity building and technical standards development.The lack of large-scale, big-ticket infrastructure and connectivity projects highlights the practical orientation of the US and Japan to “do more with less.” “Build Back Better World” (B3W) and the subsequent Partnership for Global Infrastructure and Investment (PGII) launched by the US through the Group of Seven (G7) are aimed at countering China’s Belt and Road Initiative.But for now, the American initiatives have been much cry and little wool, with tiny implementation and poor prospects.Japan has also competed fiercely with China over infrastructure construction in the region through the Partnership for Quality Infrastructure.However, it has left far behind China, especially in Indonesia, where it lost the bid for the Jakarta-Bandung High-Speed Railway project.

In the security field, the US-Japan joint efforts to strengthen military and security influence in Southeast Asia are also constrained by the US global military deployment and Japan’s “postwar system.” Although the US has nine military bases in the Philippines, there are no permanently stationed troops,and its plan to send a marine littoral regiment to the country has sparked strong controversy.In Japan, the Kishida government adopted the “three security documents” at the end of 2022 that sought to strengthen the country’s security footprint in Southeast Asia by relaxing the “three principles on transfer of defense equipment and technology” and establishing a new system for foreign military aid, but the attempt is restricted by the Pacifist Constitution and public opinions at home and abroad.As a result, the expansion of Japan’s overseas military presence will still be a long-term and gradual process.

Divergent interests and mismatched goals

The US-Japan cooperation with Southeast Asian countries involves multiple interest groups with diverse objectives, making differences in positions inevitable.Those differences can be broadly categorized into three levels.

To begin with, the US and Japan have different national interest objectives in ASEAN.First, there are subtle differences between the Japanese and US Indo-Pacific strategies.Singaporean scholar See Seng Tan pointed out that Japan tends to see the Indo-Pacific as an open platform to facilitate its expansion of influence, while the US Indo-Pacific strategy is more exclusive.There is also a gap between the two sides’ understandings of the “Free and Open Indo-Pacific.”38See Seng Tan, “Consigned to Hedge: Southeast Asia and America’s ‘Free and Open Indo-Pacific’Strategy,” International Affairs, Vol.96, No.1, pp.131-148.Second, subtle differences lie in the policy orientation of the US and Japan toward Southeast Asia.The US has a stronger ideological focus in its diplomacy toward ASEAN, while Japan values practical interests more.For example, after the sudden political upheaval in Myanmar in February 2021, the US strengthened sanctions on the Myanmar military and actively supported the opposition forces, while Japan did not cut ties with the Myanmar military and worked to maintain its traditional economic interests and influence in the country.With the prolonged political deadlock in Myanmar, Japan’s policy toward Myanmar may face pressure from the US, and the damage to Japan’s interests in Myanmar may lead to a grudge between the two countries.

Besides, ASEAN countries have different attitudes toward the US and Japan.The US treats ASEAN countries differently, which leads to significant differences in attitude and policy orientation toward the US between land and maritime ASEAN countries.On the one hand, out of the consideration of relying on the US and Japan to balance China’s influence,ASEAN welcomes the two countries to intervene in its regional affairs.This is especially the case in the Philippines, Vietnam, and other countries that have disputes with China on the South China Sea issue.On the other hand,most countries advocate that ASEAN should avoid becoming a pawn in the confrontation between China and the US, keep a proper distance from catering to the US and Japan, and strive to grasp the balance between China and the US.For ASEAN countries, sticking to the strategy of balancing between great powers is still the best choice in their policy positions.While welcoming the participation of the US and Japan in regional affairs, they are also vigilant against the great-power competition that the US and Japan promote to contain and exclude China.Notably, with the worsening situation in the Taiwan Strait, the US and Japan are attempting to contain China with the Taiwan question, and even seeking to prepare for intervention in the Taiwan Strait through the defense cooperation between them and the Philippines.The move has aroused widespread concerns among Southeast Asian countries that the conflict in the Taiwan Strait will spill over to them.In the future, if the situation spirals out of control and impacts regional stability, it may lead to a split within ASEAN countries and increase ASEAN’s annoyance toward the US and Japan.

Lastly is the goal mismatching between the US-Japan alliance and ASEAN.The essential reason for the US-Japan alliance to promote the Indo-Pacific strategy is to seek dominance in the regional order and counterbalance China’s influence, while ASEAN aims to maintain its centrality in regional affairs, and adopts the basic stand of avoiding taking sides in great-power competition.Once the US-Japan cooperation and exchanges with ASEAN emphasize too much on containing China and competing for regional dominance, ASEAN will feel uncomfortable, thus restricting the effectiveness of their cooperation.Since 2021, with the intensifying contradictions between China and the US on Taiwan-related issues and their competition in the high-tech domain, Japan has actively cooperated with the US to contain China.This not only led to Sino-Japanese confrontation, but also made ASEAN increasingly worried about the impacts of deteriorating China-US and China-Japan relations.Singapore’s Prime Minister Lee Hsien Loong has repeatedly issued public warnings about China-US relations, calling on the two countries to prevent conflicts.At the Shangri-La Dialogue held in June 2023, Singapore’s Minister for Defense Ng Eng Hen publicly stated that the most important thing Japan could do for Asian stability was improve relations with China.39“Ng Eng Hen: The Most Important Thing Japan Could Do for Stability in Asia Was to Improve Relations with China,” Lianhe Zaobao, June 4, 2023, https://www.zaobao.com.sg/realtime/singapore/story20230604-1401144.

Conceptual differences and institutional conflicts

The US-Japan alliance and the ASEAN community represent two different forms of East Asian multilateral cooperation and order with different institutional natures.40Miao Ji and Li Fujian, “US-Japan Alliance and ASEAN: The Great Competition over Asia-Pacific Order,” Forum of World Economics & Politics, No.2, 2015, pp.27-40.The former is realized through the differentiation and selection of participating actors, whereas the latter is achieved through the interconnection and integration of diverse entities.The exclusive US-Japan alliance contrasts with the pluralism and inclusiveness of ASEAN, and the basic difference naturally gives rise to a fundamental contradiction between the two.

In terms of values, the difference between the US-Japan alliance and ASEAN is manifested in the distinction between the Western democratic ideals championed by the US and Japan and the East Asian vision of coexisting multicultural traditions.The US diplomacy toward Southeast Asia has been grappling with a perpetual dilemma — the tension between its desire to promote Western democratic values and the reality of Southeast Asia’s rich diversity.Kishore Mahbubani pointed out that ASEAN has helped build a cooperative regional order with a culture of pragmatism and inclusiveness.The spirit of pragmatic cooperation helps bridge major regional political divisions, allowing most Southeast Asian countries to focus on economic growth and national development.The US should not tell countries how to run their own political systems, and Biden’s decision to frame world politics as “democracy versus autocracy” is a mistake.41Kishore Mahbubani, “Asia’s Third Way, How ASEAN Survives and Thrives Amid Great-Power Competition,” Foreign Affairs, March/April 2023, https://www.foreignaffairs.com/southeast-asia/asiasthird-way-asean-amid-great-power-competition.

Institutionally, the essential reason for the US and Japan to promote the Indo-Pacific strategy is to seek dominance in the regional order.While ostensibly claiming support for ASEAN centrality, they have used ASEAN as a tool to achieve their strategic goals, and will not truly put ASEAN in the central position regarding value orientation, mechanism construction and agenda setting.This poses a natural contradiction with ASEAN as the central framework for regional cooperation.The Quad and the IPEF promoted by the US and Japan in the region are, in fact, “non-ASEAN” arrangements,contradicting their statements of supporting the centrality of ASEAN.42Kei Koga, “Recalibrating US-Japan Indo-Pacific Strategies Towards ASEAN.”

With regard to regional integration architecture, the difference between the US-Japan alliance and ASEAN is mainly reflected in the competition over regional order between the menu-type and modular IPEF led by the US and Japan on one side and the ASEAN-centered Regional Comprehensive Economic Partnership (RCEP) and “10+3” cooperation on the other.On the one hand, as the prospects of the IPEF negotiations are still unclear, the odds are small for it to become the dominant economic cooperation framework in the Asia-Pacific region in the short term.With China and South Korea applying to join the CPTPP, which Japan and ASEAN dominate, the future regional trade and economic cooperation is expected to be driven parallelly by the RCEP and the CPTPP.The US absence will continue to limit its economic influence in ASEAN.On the other hand, the regional cooperation program launched by the US and Japan to “de-risk” has a prominent anti-Chinese orientation, and conflicts with the inclusive and open cooperation model advocated by ASEAN.In addition, China and ASEAN serve as each other’s largest trading partner with deeply integrated interests.If the US and Japan mistakenly seek to “force out” China in their cooperation with ASEAN, the ASEAN countries may not actively cooperate.

Security dilemma and risk of fragmentation

Although the US and Japan do not focus on the military in their cooperation with Southeast Asia, their joint intervention in the South China Sea issue and the layout of the US alliance system around Southeast Asia are in fact giving rise to regional security dilemma and increasing the risk of confrontation and fragmentation.ASEAN lacks a common security and defense policy, making it difficult for its member countries to seek common security by relying on collective defense.In reality, member countries have different security concerns and national interests.By pursuing the policy of balancing between great powers, ASEAN serves as a buffer zone in the competition among major powers, and the position has become the largest common denominator of security interests among ASEAN countries.The buffer space concept is also the original intention of inclusive security dialogue mechanisms such as the ARF and the ADMM+.Judging from the geo-strategic intention of the US and Japan to jointly expand in Southeast Asia, the security environment in the region faces at least three risks and challenges.

The first is the risk of camp confrontation and regional division brought by the US reinforcement of its Indo-Pacific alliance system.The US and Japan-led Quad and some minilateral security mechanisms such as the one among the US, Japan and the Philippines and the one among the US, Japan and Australia have all emphasized the strategic positioning around Southeast Asia, highlighting the orientation of containing China and flavor of camp confrontation.The US and Japan’s high-level exchanges and cooperation with Southeast Asian countries run through the global strategic consideration of confronting China and Russia, strive to promote the ideological binary narrative of “democracy versus autocracy,” and induce ASEAN countries to agree on such arguments as “safeguarding the rules-based international order”and “opposing unilateral change of status quo by force.” The moves increase the pressure on ASEAN countries to take sides and objectively challenge the ASEAN centrality and its strategy of balancing between great powers.

The second is the aggravated risk of regional division brought by the US and Japan’s involvement in the South China Sea and Taiwan-related issues.With a focus on military intervention in the South China Sea and even in the Taiwan Strait, the US and Japan have strengthened security cooperation with maritime ASEAN countries, actively assisting the Philippines and Vietnam to confront China in the South China Sea disputes, inciting the security anxiety of Southeast Asian countries on Taiwan-related issues, and taking the opportunity to reinforce their military alliance with the Philippines.By so doing, the US and Japan have dragged ASEAN into geopolitical contradictions and increased the risk of regional confrontation and conflicts.In regional security affairs, the different attitudes adopted by the US and Japan toward land and maritime ASEAN countries have damaged the integration process of ASEAN and interfered with ASEAN’s cohesion.

The third is the risk of nuclear proliferation and arms race brought by the trilateral security partnership among the US, the United Kingdom and Australia (AUKUS).The AUKUS cooperation on nuclear-powered submarines has triggered alarm and opposition from some ASEAN countries.On the one hand, the competition over norms and order between the exclusive and minilateral AUKUS and the ASEAN-centered regional cooperation architecture may challenge the normal guarantee that is vital for ASEAN countries’ respective and collective survival.43Xue Liang and Zheng Xianwu, “ASEAN’s Perceptions of and Responses to AUKUS: Tension between Minilateralism and Regional Multilateralism,” Global Review, No.2, 2023, pp.64-65.On the other hand, the transfer of nuclear submarine technology, which is the current focus of AUKUS, might break the regional military balance, inducing a new round of arms race in the Asia-Pacific, undermining the construction of a nuclear-free zone in Southeast Asia, and eventually blocking the political and security integration process of ASEAN.44“Chinese Ambassador to ASEAN Deng Xijun: AUKUS Will Bring Five Dangers to the Region,” The Paper, November 25, 2021, https://www.thepaper.cn/newsDetail_forward_15552663; Chen Xiangmiao,“AUKUS Poses Three Challenges to ASEAN,” China Daily, September 27, 2021, https://column.chinadaily.com.cn/a/202109/27/WS61516b7fa3107be4979f002e.html.As the US promotes the expansion of AUKUS membership, Japan and the Philippines’ cooperation under the AUKUS framework in the future may impact the unity of ASEAN.

Conclusion

The collaborative expansion of the United States and Japan in Southeast Asia are transitioning from ad-hoc spontaneous coordination to deliberate strategic synergy, in order to enhance their political, economic, security, and cultural influence in the region.Relying on Japan’s economic and cultural advantages formed by its years of deep cultivation in Southeast Asia, the US has strengthened its all-round engagement in the region and grassroots outreach.In return, Japan has benefitted from the security umbrella provided by the US military influence, which addresses the gap in its Southeast Asia diplomacy.Despite some positive outcomes in the US-Japan joint expansion in Southeast Asia, significant disparities still exist in their values and interests as well as their preferred institutional arrangements.Additionally, ASEAN is reluctant to blindly side with the US and Japan given its strategy of balancing between great powers.Consequently, the impact of the US and Japan on Southeast Asia will remain constrained by the level of their investment and policy alignment.

China and Southeast Asia share close geographical proximity and maintain deep economic, trade, and cultural connections.These multiple linkages imply that China possesses advantages unavailable to the USJapan alliance.Whether China can gain the upper hand in Southeast Asia hinges on its capacity to offer regional public goods, its level of support for Southeast Asian nations, and the resolution of the South China Sea issue.To this end, China should firmly prioritize ASEAN in its neighborhood diplomacy, increase its resource input, strive to upgrade the quality of China-ASEAN trade and economic cooperation, resolutely support ASEAN’s centrality and strategic independence, and make more efforts to stabilize the situation in the South China Sea.