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. , The US relies heavily on its allies in the region, especially the Philippines and Japan, for access to Indo-Pacific waters, intelligence sharing, and military capabilities to counter China’s , New defence cooperation initiatives from the United States and its allies focus largely on the Philippines, Indonesia, and Singapore. This trend risks leaving mainland Southeast Asia more reliant on cooperation with China and Russia, increasing the geopolitical divide within the Association of Southeast Asian Nations (ASEAN)..