Southeast Asia’s evolving defence partnerships

Southeast Asian countries have diversified their defence partnerships amid intensifying strategic competition and regional security challenges. While the United States and China remain key security actors, countries in the region are engaging with a broader array of external partners to enhance their autonomy and military capabilities. Australia, Japan, India, and South Korea are all important defence partners for the region, offering capacity-building, training, and technology cooperation. These partnerships reflect Southeast Asia’s search for resilience through engagement with multiple partners while avoiding over-dependence on any single actor.

Since the end of the Second World War, the United States has been the key defence partner for most Southeast Asian countries. It is a treaty ally of the Philippines and Thailand, and a close strategic partner to Singapore. It maintains a high tempo of defence engagement through combined military exercises and strategic and service-level defence dialogues across the region. These activities foster interoperability, build trust, and expose Southeast Asian countries to advanced military tactics and technology.

Yet the landscape for defence cooperation in Southeast Asia is becoming more complex and contested as geopolitical competition rises. China is working to replace the United States’ security role and influence in Southeast Asia. To this end, Beijing has strengthened defence cooperation with most Southeast Asian countries, albeit from a low base. It has found most success in mainland Southeast Asia, where it has become a key defence partner for Cambodia and Laos.

Middle powers such as Australia, India, and Japan have also strengthened their defence ties with Southeast Asia. Several factors drive this. All three are heavily trade-dependent, so they seek to protect freedom of navigation and overflight routes through the region. Defence ties with Southeast Asian countries can help them maintain regional presence, situational awareness, and collaborative partnerships. Relatedly, these external partners all share concerns about China’s growing influence in Southeast Asia and see defence ties as one means by which to maintain a regional balance and shape regional rules and norms.

Defence cooperation has many elements, which vary by country. For example, Vietnam participates in relatively few combined military exercises but is an active participant in dialogues; Russia is an important arms procurement partner for Southeast Asia but is less active in exercises and dialogues. The main dimensions of defence cooperation assessed in this paper are defence agreements, defence dialogues, and combined military exercises with ten external partners: Australia, Canada, China, France, India, Japan, Russia, South Korea, the United Kingdom, and the United States. Three case studies are covered: Malaysia, Indonesia, and Thailand. The first two are in maritime Southeast Asia while the latter is mainland. These case studies offer a more detailed analysis of the dynamics shaping defence cooperation between regional states and the ten external partners. Indonesia is the largest state in Southeast Asia, with critical maritime waterways such as the Malacca and Sunda Straits. Malaysia is a member of the Five Power Defence Arrangements (FPDA), which include Australia, while being a claimant in the South China Sea disputes. Thailand is an ally of the United States, but with China as a major arms supplier.

Our research draws on both quantitative and qualitative data, the former drawn from a broader program of work at the Lowy Institute as part of the Influencing Southeast Asia Project, the latter drawing on interviews with experts and officials from Southeast Asian countries. The full findings of the Influencing Southeast Asia Project, which covers five dimensions of influence (defence partnerships, diplomatic influence, economic relationships, cultural influence and regional engagement), will be published in a separate digital interactive. Data presented in this paper covers the period 2017–2024. [1]

Top external partners

Based on the number of defence agreements, dialogues, and combined military exercises, the United States is the top overall defence partner for Southeast Asia among the ten selected external partners, while China ranks eighth. Russia is second bottom of the list (see Figure 1a). However, a further breakdown of the data reveals a divergence between mainland Southeast Asia (Cambodia, Laos, Myanmar, Thailand, and Vietnam) and maritime Southeast Asia (Brunei, Indonesia, Malaysia, Philippines, Singapore, and Timor-Leste). The United States is not the top defence partner in mainland Southeast Asia.

China is the top partner for Laos and Cambodia. India and Australia hold the top spot in Myanmar and Malaysia, respectively (see Figure 1b). While maritime Southeast Asian countries have experienced growing interest from external defence cooperation partners, the same is not true for mainland Southeast Asia (see Figure 1c). The United States has reduced defence engagements with Cambodia since the 2010s and with Thailand since the 2014 coup. Thereafter, China stepped up defence engagements with these two countries.

Defence agreements

Data comparing 2017 and 2024 shows the growth in defence agreements between Southeast Asian countries and external partners. Every one of the ten external partners has established additional agreements with Southeast Asian countries over that period (see Figure 2a).

This surge has not been monopolised by the United States and China. Indo-Pacific middle powers are engaging with Southeast Asian countries too. Collectively, Quad members Australia, India, and Japan have signed more defence agreements with Southeast Asian countries than China and the United States combined. The former have signed 38 new defence agreements, compared to 22 by the United States and China (see Figure 2b). If Canada and South Korea are included, the collective figures for the middle powers dwarf those of the United States and China (see Figure 2b).

Since the 2021 military coup, Myanmar has been comparatively isolated in defence relations with external partners. While nearly all other ASEAN countries have signed more defence agreements with external partners since 2017, Myanmar has not signed any with the ten external partners tracked here. In fact, the defence agreement with Australia (the Defence Cooperation Program) was suspended in 2021 following the military coup. At the other end of the spectrum, Indonesia and the Philippines are the Southeast Asian countries with the highest number of defence agreements, reflecting their importance to external partners (see Figure 2c).

The quality and strategic value of defence agreements in Southeast Asia varies considerably. Agreements with the United States and Japan stand out for their practical impacts. The Acquisition and Cross-Servicing Agreement (ACSA) that the United States has signed with Malaysia, Singapore, and Thailand covers logistical support, supplies, and equipment used during exercises between American forces and their Southeast Asian counterparts. For example, Malaysian forces participating in military exercises alongside American forces such as RIMPAC are supported with food and fuel through the bilateral ACSA. [2] Japan’s series of agreements on the transfer of defence equipment and technology allows it to export defence technology and equipment in line with its goal of bolstering security and resilience in Southeast Asia. [3] For example, Japan provided Coastal Radar Systems to the Philippines through the Official Security Assistance (OSA) arrangement introduced in 2022. [4]

Beyond hardware, agreements to enable the sharing of classified information and technology reflect trust between the signatories. For the United States, the General Security of Military Information Agreement (GSOMIA) provides a framework for such sharing. The United States has signed GSOMIAs with the Philippines, Singapore, and Thailand, institutionalising the sharing of intelligence and technology, and thus strengthening defence ties with these three countries.

In contrast, China’s defence agreements with countries such as Brunei, Malaysia, Singapore, Thailand, and Vietnam are mostly vague and symbolic, containing only general commitments to cooperation and dialogue. Some may include academic exchanges and professional military education, such as the one between Singapore and China. [5] However, they lack substantive provisions for technology transfer, combined training, or intelligence sharing. The 2023 Brunei–China Memorandum of Understanding, for example, resulted in the creation of a Joint Defence Working Committee whose activities have been largely superficial, centred on goodwill gestures such as museum tours and facility visits rather than fostering genuine military interoperability or strategic trust. [6]

Defence dialogues

Our data tracks both the establishment of formalised mechanisms and the occurrence of defence dialogues. Since 2017, the ten external partners have set up additional dialogue mechanisms with Southeast Asian states, demonstrating a deepening engagement with the region (see Figure 3a). However, the majority of these engagements are concentrated on the Philippines, Vietnam, Indonesia, Malaysia, and Thailand (see Figure 3b).

While the United States is the leading partner in terms of both dialogue mechanisms and number of dialogues convened, there is a discrepancy for other partners. For example, India had the second-largest number of institutionalised defence dialogue mechanisms in 2024 (26) but ranked just fifth in terms of the number of dialogues convened (64) over 2023 and 2024 — an example of the “say-do” gap often noted in relation to India’s Southeast Asia engagement.

Among Southeast Asian countries, the Philippines has the most dialogue mechanisms with the ten external partners (27). Vietnam is in second place (25), and Indonesia is third with 23. However, the quality of the dialogue mechanisms matters. For example, Australia and the United States maintain extensive and multifaceted dialogue channels with several regional countries, holding service-level staff talks (Army, Navy, or Air Force) with seven countries: Brunei, Indonesia, Malaysia, the Philippines, Singapore, Thailand, and Vietnam. These engagements serve as key avenues for operational and tactical-level interactions. Indonesia stands out as the only Southeast Asian country to engage in service-level dialogues with both China (Army Staff Talks) and Russia (Navy Staff Talks).

Combined military exercises

Combined military exercises serve multiple purposes. At the higher end of the spectrum, they can facilitate interoperability between Southeast Asian forces and their external partners, allowing partners to practice operating together, standardise procedures, and integrate command-and-control systems for joint missions. [7] At the level of political signalling, they may reinforce alliances or alignments, demonstrating commitment to shared security interests. Indonesia, for example, conducts exercises with China and Russia to demonstrate its non-aligned stance. [8] For external powers, exercises can also be a means to reinforce presence and situational awareness in Southeast Asia.

At the lower end of the spectrum, combined military exercises can foster mutual trust and mitigate tensions. They may also promote transparency by sharing differing procedures and protocols, reducing the scope for misinterpretations and accidental escalation.

Among the ten external partners, the United States is the most active in combined military exercises in Southeast Asia. It was involved in around 40 per cent of all bilateral or multilateral combined military exercises involving Southeast Asian countries (161 out of 369) in 2023 and 2024. The next two places are occupied by Washington’s treaty allies in the Indo-Pacific region — Japan (134 exercises) and Australia (104). Russia occupies the bottom place among countries covered by our research (see Figure 4a). China is more active in mainland Southeast Asia, while the United States is primarily focused on combined military exercises with partners from the region’s maritime countries (see Figure 4b).

Indonesia, the Philippines, and Singapore are the top three Southeast Asian countries when it comes to conducting combined military exercises with the ten external partners. Laos, Timor-Leste, and Myanmar take the bottom three places (see Figure 4c).

When looking at individual Southeast Asian countries, Australia, Japan, and the United States are among the top three exercise partners in 8 out of the 11 countries (Brunei, Indonesia, Malaysia, Myanmar, the Philippines, Singapore, Thailand, and Timor-Leste). The United States is the top exercise partner for five of them (Indonesia, the Philippines, Singapore, Thailand, and Timor-Leste). Japan is the top partner for six other Southeast Asian countries (Brunei, Cambodia, Malaysia, Myanmar, Timor-Leste, and Vietnam). China is the top exercise partner for Cambodia and Laos (see Figure 4d), demonstrating the extent of China’s reach with these two countries.

Military exercises with the United States have expanded in both scale and complexity, with many evolving from bilateral to multilateral operations. The Garuda Shield exercise, launched in 2007 as a modest bilateral army-to-army drill with Indonesia, grew by 2024 into a multi-domain exercise involving land, air, and maritime components with forces from 10 participating countries and 12 observing countries. [9] Similarly, the 2024 Balikatan exercise with the Philippines featured 16,000 personnel conducting multi-domain operations alongside American allies, including Australia and France. [10] These exercises increasingly emphasise combat readiness and interoperability among participating forces, but also coordinating cyber defence. Figure 5 provides more information on the qualitative expansion of these exercises over time.

The military exercises of Southeast Asian countries with middle powers such as Australia and the United Kingdom follow the same trend as those with the United States, with a focus on interoperability and combat operations. Malaysian and Singaporean military forces participating in the Five Power Defence Arrangements (FPDA) exercises alongside their Australian, British, and New Zealand counterparts are exposed to high-tempo and realistic battle scenarios. They operate through uniform communication channels and a joint command post, with different forces having to operate together in an environment close to a wartime situation. [11]

China has ramped up military exercises with Southeast Asian countries. However, these exercises largely remain limited to non-traditional security domains such as counter-piracy, counter-terrorism, and humanitarian assistance. For instance, Exercise Cooperation with Singapore in 2023 and 2024 focused on counter-terrorism operations, a theme echoed in the 2025 Cambodia–China Golden Dragon exercise. [12] Similarly, Laos–China drills, such as Friendship Shield 2024, remain relatively basic, prioritising technical coordination and joint responses to transnational crime rather than advanced combat capabilities or high-end interoperability. [13]

Interoperability is conspicuously lacking in China’s military exercises with regional partners, a reflection of Beijing’s reluctance to expose its capabilities, and differences in systems and doctrines. China’s cautious stance has in turn bred mistrust. One Malaysian naval official observed that Chinese naval personnel involved in exercises with other partners tended to avoid addressing queries on the capabilities of Chinese warships. The official noted, “The more we do exercises with them, the more distrust we have against the Chinese.” [14]

Convergence of security interests

The data indicates that defence cooperation between Southeast Asian countries and external partners has increased in recent years through the establishment of defence agreements, more frequent defence dialogues, and combined military exercises. This intensification is shaped by a convergence of interests driven by two broad factors: demand from Southeast Asia and the interests of external partners.

Demand side — Southeast Asian countries

Southeast Asian countries are increasingly aware of the limitations of their military capabilities, and many face challenges in modernising their armed forces within their budgets. External defence partnerships offer access to advanced technology, training, and logistical support that would otherwise be difficult to obtain. For example, a trilateral exercise between Australia, Malaysia, and the United States in 2024 enabled Malaysia to test its airlift capabilities in support of military operations in the eastern Malaysian state of Sabah, which faces the South China Sea. [15] Indonesia relies on defence partnerships with external actors to access advanced military technology. Jakarta recently renewed its participation in the South Korean KF-21 combat aircraft project, which was impacted earlier by Jakarta’s tight fiscal situation. [16] In this case, interest in procurement, rather than a broader strategic objective, drives Indonesia’s choice of South Korea as an important partner.

China is one reason why regional countries seek to modernise and expand their armed forces’ capabilities. Within Southeast Asia, there is a growing concern about China’s assertiveness, particularly in the South China Sea. China’s expansive maritime claims and militarisation of artificial islands have alarmed Malaysia, the Philippines, and Vietnam. [17]

These concerns have prompted Southeast Asian countries to seek closer defence ties with external powers such as Australia, Japan, and the United States to bolster their strategic position and deter aggression. The Philippines has confronted China’s encroachment in its exclusive economic zone (EEZ) in the South China Sea. [18] But given its weak defence capabilities, Manila has sought greater assistance from external partners, including Australia, Japan, and South Korea, as well as its treaty ally, the United States. [19] Malaysia considers China’s behaviour in the South China Sea and its expanding military capabilities as primary security concerns. [20] Malaysia’s defence establishment aims to develop additional naval infrastructure while conducting joint military exercises with Australia and the United States in East Malaysia, which faces the South China Sea. [21] However, while the Philippines has sought to garner international support through exercises such as a series of maritime cooperative activities in the South China Sea, Malaysia’s approach is quieter. Even as Malaysia’s political leaders seek closer ties with China, the Malaysian defence establishment still values ties with external partners as a way to boost military capabilities in East Malaysia. [22]

Another factor driving regional states to seek defence cooperation with external partners is the persistence of perceived threats from their immediate neighbours, as demonstrated by deadly border skirmishes between Cambodia and Thailand in 2025. These clashes, like others throughout their shared history, will likely heighten Cambodia’s concerns about perceived threats from Thailand and Vietnam, and drive further defence cooperation with China. [23] Another example is Singapore, where security concerns regarding Malaysia and Indonesia are likely one factor driving close defence relations with the United States. [24]

Southeast Asian countries seek breadth as well as depth in their international partnerships, making the role of middle powers crucial. Working with a wide range of countries can help maintain autonomy and avoid reliance on any one partner. This motivation has led the Philippines to sign a new reciprocal access agreement with Japan and step up its defence cooperation with Australia. Working with these partners can also be less sensitive, as it may avoid the perception of taking sides in the US–China rivalry. Vietnam, for example, is cautious to avoid the perception of siding with the United States but is less worried about this perception when it comes to Australia or Japan. In 2024, Vietnam sent a naval vessel to Australia for an exercise, the first such visit to a Western country. [25]

Supply-side — external partners

Southeast Asia’s maritime geography, situated at the fulcrum between the Pacific and Indian Oceans and straddling key waterways and straits, gives major powers a direct interest in security cooperation with the region, as part of a broader contest for influence. This contest has intensified since the start of the 2010s, with China’s growing regional influence and more assertive foreign policy. The Chinese campaign of artificial island building in the South China Sea pushed the United States and its allies to work harder to maintain a regional balance.

For the United States, the increasing size, length, and complexity of flagship exercises such as Balikatan with the Philippines and Garuda Shield with Indonesia signal its commitment to maintaining a presence within the “first island chain” — extending from Japan, Taiwan, portions of the Philippines, and Indonesia, forming a line that separates China from the Pacific Ocean. [26] Over the years, these exercises have attracted a more diverse range of participants while also emphasising interoperability through joint operations and the introduction of more advanced weapons systems.

Japan and Australia have contributed to these efforts by participating in flagship US-led combined exercises. But they have accelerated their independent roles too — Australia as a key exercise and dialogue partner, and Japan as a provider of official security assistance, coastguard cooperation, and partnerships on maritime security. The goal of these activities is often implicitly framed as promoting regional resilience to the risk of coercion by China. To a lesser extent, other US allies such as Canada (in the case of its partnership with the Philippines) have been motivated by the priority of supporting Washington’s Indo-Pacific strategy.  [27]

For Russia, India, and South Korea, defence industrial objectives play a greater role in driving defence partnerships. [28] India’s defence dialogues with Malaysia and Thailand include a focus on selling and sharing defence equipment and manufacturing capabilities, and India has had some success with defence sales to the Philippines. [29] South Korea’s defence dialogues with Southeast Asia are also seen as a pathway to arms sales, as demonstrated by Vietnam expressing its interest in South Korean artillery systems during a recent bilateral defence meeting. [30]

Russian influence in Southeast Asia is waning. It was the region’s largest established arms supplier until around 2018. According to the Arms Transfers Database from the Stockholm International Peace Research Institute (SIPRI), Russia’s share of the Southeast Asian arms market has dropped from as high as 30 per cent before 2014 to below 10 per cent since 2018. On the other hand, China and the United States have seen their shares of the Southeast Asian arms market grow. [31] Nevertheless, Russian President Vladimir Putin and the Russian political elites view Russia as a great power that the international community should respect. [32] As a result, it seeks to challenge Washington’s dominant security role in Southeast Asia and sees the United States as a stumbling block to its global ambition. [33]

The findings of our study suggest that China seeks security cooperation with Southeast Asian countries as part of its broader efforts to compete with the United States in Asia, an example of what Oriana Skylar Mastro has referred to as China’s strategy of “emulating” the United States. [34] For example, China has opportunistically strengthened its defence ties with Cambodia and Thailand following the cooling of both countries’ defence relations with the United States. [35] Its expansion of a bilateral exercise with Malaysia, Aman Youyi, into multilateral exercises, including four other countries, also emulates the “multilateralisation” of major US exercises in the region. [36]

Other secondary motives may also play a role, including the opportunity to learn from and train alongside Western-equipped forces, for example, in the case of defence cooperation with Thailand (see case study below). Finally, as other scholars have argued, China may have interests in working on internal security matters with its partners, which at times, for example in relation to counter-terrorism, can overlap with traditional defence cooperation on external security issues. [37] Internal security cooperation lies outside the scope of this research.

Divergence in Southeast Asia

The increase in defence cooperation between Southeast Asia and its external partners has not been uniform. The countries most involved in new defence cooperation initiatives since 2017 are in maritime Southeast Asia, particularly the Philippines, Indonesia, and Singapore. For example, of 51 new dialogue mechanisms established since 2017, more than 80 per cent were with maritime Southeast Asian countries and Vietnam.

Most external partners, including the United States and its allies, emphasise the importance of geopolitical contestation in the maritime domain. [38] In different ways, the Philippines, Indonesia, Singapore, and Malaysia each occupy key strategic geography that is of interest to the region’s external partners. Likewise, it is maritime countries that have tended to worry most about China’s rising regional influence, particularly in the context of China’s military build-up and coercive tactics in the South China Sea. The drive for them to seek a diversity of external security partners is therefore stronger than among countries of mainland Southeast Asia.

The diversity of Southeast Asian countries’ approaches to external security partnerships is highlighted by case studies on Indonesia (the region’s largest and most consequential country), Malaysia (a country with close political ties to China but traditional security ties with Australia and the United States), and Thailand (a US ally that is deepening defence ties with China).

MALAYSIA

Malaysia’s multifaceted defence partnership strategy serves several goals. First, cooperation with external partners is seen as strengthening Malaysia’s defence posture, given its military’s low capabilities. [39] Second, defence cooperation with the United States and China contributes to the stability of Malaysia’s strategic environment. China is Malaysia’s primary security concern, and the United States plays a critical role in balancing against this perceived threat; [40] defence partnerships enable Malaysia to maintain open lines of communication with both while avoiding entangling alliances. Third, defence cooperation with a broad spectrum of partners reinforces Malaysia’s non-aligned foreign policy outlook, which explains Malaysia’s preference to work with middle powers such as Australia and Japan. [41]

Figure 6a demonstrates Malaysia’s approach towards defence cooperation. While cooperation with the United States is substantial, with 39 combined military exercises conducted at the bilateral or multilateral level, Malaysia has also engaged Australia and Japan.

Traditional defence partners — Five Power Defence Arrangements and the United States

Australia, the United Kingdom, and the United States are Malaysia’s traditional security partners. Defence cooperation with Australia, the United Kingdom, New Zealand, and Singapore is formalised through the FPDA, established in 1971. This pact commits members to consult one another in the event of an armed attack on Malaysia or Singapore. FPDA exercises such as Bersama Shield and Bersama Lima are critical for developing Malaysia’s military capacity. As Malaysia lacks robust joint operations capabilities, these exercises — its only regular multinational engagements — offer rare opportunities to operate alongside more advanced partners like Australia and the United Kingdom. They expose Malaysian personnel to modern tactical and operational practices, which directly enhance their ability to integrate with other forces, including in United Nations peacekeeping missions. [42]

Beyond the benefits of exercises with the United States, defence relations with Washington allow Putrajaya to extract technical and material aid from Washington to enhance Malaysian military capabilities. In 2021, Washington awarded a contract to Lockheed Martin to supply Malaysia with a long-range radar under the Building Partner Capacity program. [43] This radar, deployed in East Malaysia, enables Malaysia to monitor up to 470 kilometres into the South China Sea. Other forms of assistance include helping Malaysia to build airlift capabilities through Exercise Keris Strike 2024, and the 2025 approval to transfer and upgrade the software of Malaysian F/A-18 combat aircraft. [44]

Emerging defence partners

New Delhi and Putrajaya hold defence dialogues at strategic and service-to-service levels. The Malaysia–India Defence Cooperation Committee (MIDCOM) serves as a platform for senior defence officials to discuss defence cooperation and for Malaysia to receive assistance and maintain its military readiness. The latest MIDCOM meeting in February 2025 resulted in an agreement to share expertise and best practices in maintaining Russian-made Su-30 combat aircraft, which both operate. [45]

The Malaysia–Japan defence partnership led to Tokyo providing practical assistance to Putrajaya in the maritime domain. The Exchange of Notes for the Official Security Assistance (OSA) agreement with Japan, signed in 2023, saw Japan promising to provide rescue boats to Malaysia to boost its maritime capabilities. [46]

China has also stepped up defence engagements with Malaysia. In 2025, Malaysia and China agreed to establish a 2+2 defence dialogue mechanism involving defence and foreign ministers. [47] Since the two parties have competing maritime territorial claims in the South China Sea, Malaysia sees institutionalising bilateral dialogues with China as helping to manage this complex relationship. These efforts have borne some fruit — Malaysia and China have reached an informal and unwritten understanding regarding their overlapping maritime claims in the South China Sea, which enables the former to extract energy and other maritime resources without facing physical harassment from Chinese naval forces. [48]

Exercises with China serve little military purpose; they are generally unsophisticated and small-scale, with few tactical lessons for the Malaysians. [49] But they do demonstrate Malaysia’s stance as a non-aligned actor. Such exercises are infrequent, with the last bilateral one (Aman Youyi) held in 2016, after which they evolved into multilateral ones involving other ASEAN members, starting in 2018. [50]

INDONESIA

Jakarta’s approach to defence diplomacy emphasises strategic autonomy and regional stability. President Prabowo Subianto frequently quotes the proverb, “A thousand friends are too few, one enemy is too many.” This is reflected in the broad range of defence agreements Indonesia has signed with partners, within and outside Southeast Asia.

Defence activities with external partners, particularly China and Russia, demonstrate that Indonesia remains a non-aligned actor in an increasingly contested Indo-Pacific region. [51] For example, in July 2024, the Indonesian Army Chief of Staff, General Maruli Simanjuntak, announced that Jakarta planned to conduct more military exercises with China and ASEAN countries. [52] This came shortly before then Indonesian Defence Minister Prabowo Subianto visited Canberra in August 2024 to finalise the Australia–Indonesia Defence Cooperation Arrangement. [53]

Another driver influencing Indonesia’s approach to defence partnerships is Jakarta’s interest in acquiring advanced military technology. Technology transfer from more advanced partners is crucial for Jakarta to achieve its goal of increasing the domestic content level in defence and security equipment from the current 40 per cent to 70 per cent. [54] Indonesia–France defence relations provide one example. After the two countries signed a defence cooperation agreement in 2021, defence engagements expanded, with more frequent dialogues facilitating cooperation between defence firms on the joint production of military equipment. [55] In 2024, they announced that two new Indonesian submarines would be built in Indonesia through the transfer of technology from the French Naval Group. [56]

Russia has a strong defence partnership with Indonesia rooted in deep historical connections. The Soviet Union supplied Indonesia with military equipment in the late 1950s. From 1961, Moscow began a massive re-equipping of the Indonesian National Armed Forces, providing it with tanks, submarines, warships, and combat aircraft and training thousands of personnel. [57]

Over the past 15 years, Indonesia has signed various defence agreements with Russia, ranging from technical cooperation to military personnel exchanges and joint military exercises. In September 2010, a Russia–Indonesia agreement was signed to strengthen military-technical cooperation. Defence cooperation was further expanded when the two parties signed the 2016 Cooperation Agreement on Joint Training, Exercises, Officer Exchanges, and Technological Cooperation, and an Agreement on Cooperation in International Information Security in 2021. Russia has an interest in gaining a foothold in Southeast Asia, but its reported efforts to gain access to an Indonesian air base near West Papua remain unsuccessful so far. [58]

In balancing Russian influence, Indonesia maintains a robust defence partnership with the United States. Jakarta has signed several defence agreements with Washington, notably a Defence Cooperation Agreement in 2023. At the strategic level, defence dialogues are held in the form of the Indonesia–United States Security Dialogue and the US-Indonesia Bilateral Defence Dialogue. At the service level, the two parties’ armies, navies, and air forces hold annual talks, which are supplemented by Submarine Force Staff Talks to exchange best practices on submarine force tactics and crisis response, as well as incorporating tabletop exercises. [59]

The United States is a key exercise partner for Indonesia in both quantitative and qualitative terms. Between 2023 and 2024, Indonesia and the United States participated in more than 50 bilateral and multilateral exercises. Generally, these exercises are of high quality and emphasise interoperability and combat readiness. Besides the Garuda Shield and Super Garuda Shield exercise series, Exercise Cope West is an air exercise between the American and Indonesian air forces aimed at enhancing interoperability and sharing of techniques. [60]

China has stepped up its defence engagements with Indonesia in recent years, which have traditionally been at a low level. They agreed in 2024 to commence two high-level dialogue mechanisms — a ministerial 2+2 dialogue involving defence and foreign ministers, and a senior-officials level 2+2 meeting.

Despite having an overlapping maritime claim with China around the Natuna Islands in the southern part of the South China Sea, Indonesia conducts military exercises with China. These exercises demonstrate Indonesia’s effort to maintain defence ties with all regional players and an unwillingness to choose sides between the United States and China. However, exercises with China remain limited in terms of both number and quality, with a focus on non-traditional security. For example, the China–Indonesia Heping Garuda-2024 exercise focuses on natural disaster rescue operations. [61] While these exercises are less sophisticated than US–Indonesia exercises, Indonesia likely still values cooperation on disaster relief, a priority for its armed forces.

Australia and Indonesia signed a Defence Cooperation Agreement in 2024, while defence engagements have expanded in recent years. Australia participated in Garuda Shield, a formerly bilateral Indonesia–US military exercise, for the first time in 2022. [62] A year later, the Indonesian military participated in Exercise Talisman Sabre in Australia for the first time. [63]

THAILAND

Thailand is one of two US treaty allies in Southeast Asia. Thai defence cooperation continues to heavily emphasise the United States, with fellow US ally Australia being its second-most important external security partner.

Cobra Gold, hosted by Thailand, is the region’s largest and longest-running multilateral military exercise (Figure 8). In 2025, it involved around 30 countries and 3,200 US personnel, though the scale of US participation has declined since the mid-1990s. [64] Yet despite its size, compared to the trajectory of other US flagships such as Balikatan or Garuda Shield, which have become markedly more ambitious in line with US priorities, Cobra Gold has remained largely focused on humanitarian and disaster response activities, reflecting the relative lack of dynamism in the Thailand–US defence partnership.

Thailand’s intense participation in 49 combined exercises with the United States over the 2023–2024 period reflects a high level of interoperability and familiarity between the Thai and US militaries. And while Bangkok lacks the same sense of strategic alignment with Washington as now exists between the United States and the Philippines, the two countries are still heavily engaged in a program of talks at both the strategic level (a new defence strategic dialogue was initiated in 2022) and operational level (for example, service exchanges). Bangkok also highly values US professional military education. [65]

Yet Thailand operates a non-aligned foreign policy, with the alliance having declined in relevance for both sides over recent years. Especially since the 2014 military coup, several signals point to a shift in Thailand’s external defence partnerships. The United States strongly criticised the coup, froze $4.7 million worth of security assistance, and relations were not normalised until 2017. This created an opportunity for China to significantly expand its defence ties with Thailand, a trend most noticeable when it comes to defence procurement. Over the ten years to 2017, the United States was the top source of defence equipment for Thailand, accounting for 12 per cent of the total. Over the ten years to 2025, US defence sales stayed roughly the same but China became the top-ranked source of imports, accounting for 29 per cent of the total.

Not all defence cooperation pledged between China and Thailand in the wake of the 2014 coup has come to pass. A deal to purchase three Chinese submarines has faced extensive delays and complications, and after more than a decade it is unclear whether the purchase will go ahead. Other mooted cooperation, such as an agreement for China to establish in Thailand a facility to produce, assemble, and maintain Chinese land-based weapons systems, has also not come to pass.

One area where Thailand and China have increased their defence cooperation significantly is in the field of bilateral combined exercises. For example, the annual bilateral air force exercise, Falcon Strike, was, in 2015, a three-day exercise in which Chinese fighter pilots were outmatched by their Thai counterparts. [66] By 2024, it had expanded to an 11-day exercise involving 13 types of People’s Liberation Army Air Force (PLAAF) aircraft and ground-based air defence systems. While some analysts have attributed a defence industry motive to China’s prioritisation of this exercise, another likely factor is that exercising with Thailand provides China with the rare opportunity to exercise alongside a Western-trained military force. Noting these sensitivities, Thailand does not fly the US-built F-16 or F-5 fighters in exercises with China. [67]

It is notable that unlike countries in maritime Southeast Asia, Thailand is yet to reap the full benefit of Japan’s expanded security cooperation with Southeast Asia. In 2022, Japan and Thailand signed an agreement to facilitate the transfer of defence hardware and technology, but Thailand has not received any defence equipment from Japan over the past ten years. [68] This stands in contrast to Malaysia, the Philippines, and Indonesia, which have already received equipment. [69]

Amid intensifying strategic competition, Southeast Asian countries are reshaping their defence engagement strategies to safeguard their autonomy and bolster regional stability. The United States remains a central security partner, especially in advancing military interoperability; Southeast Asian countries’ defence engagements with the United States and its allies are generally deep and high-quality, reflecting strategic trust. And while the Trump administration’s aid and tariff policies are affecting broader US engagement with Southeast Asia, defence engagement has been characterised by continuity rather than change.

The research suggests that China has invested substantial effort in establishing greater defence cooperation with Southeast Asian countries, and that it likely intends to emulate and compete with the United States in this domain. And while defence engagements with China remain at a superficial level for now, Beijing has made steady progress, particularly in mainland Southeast Asia. Further inroads, especially if nascent defence cooperation with Indonesia and Malaysia were to become more substantial, would add to China’s narrative and provide it with new pathways to pursue its interests.

At the same time, middle powers such as Australia, Japan, and South Korea have become increasingly important, offering practical support with less geopolitical baggage than the region’s great powers. This diversification of defence partnerships illustrates the region’s approach: leveraging multiple relationships to strengthen resilience and reduce strategic dependence.

Yet this research demonstrates that the benefits of partnerships with the United States and middle powers have not been evenly distributed across Southeast Asia, with the Philippines, Indonesia, and Singapore attracting the largest share of new defence cooperation initiatives, reflecting the strategic importance of maritime Southeast Asia. This trend risks exacerbating ASEAN divisions between mainland and maritime Southeast Asian states. If current trends persist, Southeast Asia risks dividing into two camps: maritime states with deep defence ties to the United States and its allies, and mainland states lacking such cooperation. External partners seeking to engage with Southeast Asia should be mindful of this trend, and where possible, look for pathways to include the region’s smaller countries, such as Laos and Cambodia, in defence cooperation initiatives. If they do not, China and Russia will continue to entrench themselves as the predominant security partners for these countries.

This research was supported by the Australian Government through a grant by the Department of Defence. The views expressed herein are those of the authors and are not necessarily those of the Australian Government or the Department of Defence. 

The authors acknowledge the editorial and design contributions of Ian Bruce, Clare Caldwell, and Sam Roggeveen, as well as the helpful comments by two anonymous peer reviewers.