In a research report titled Southeast Asia’s Evolving Defence Partnerships, published by the Lowy Institute (2025) and authored by Rahman Yaacob, Susannah Patton, and Jack Sato, the authors analyse the shifting dynamics of defence cooperation in Southeast Asia amidst intensifying regional security challenges and growing global strategic competition. The report examines how Southeast Asian countries are diversifying their defence partnerships with external powers such as the United States, China, Australia, Japan, India, and South Korea to enhance their military capabilities and safeguard their sovereignty without being overly reliant on any single partner.
Historically, the United States has been the predominant defence partner for most Southeast Asian countries. It is a treaty ally of the Philippines and Thailand and a close strategic partner of Singapore. However, in recent years Southeast Asian nations have diversified their defence ties, aiming to enhance regional security, access advanced military technology, and strengthen defence capacities through joint training, technology transfers, and logistical support. The authors argue that while the United States remains a key security partner, China has significantly expanded its defence presence, particularly in mainland Southeast Asia, though its agreements are often more symbolic compared to those of the United States and its allies. Some of China’s agreements include provisions for academic exchanges and professional military education, but they lack the substance of joint training, technology transfers, or intelligence-sharing arrangements.
Furthermore, the report identifies key trends and drivers influencing defence cooperation. Southeast Asian nations are increasingly aware of the challenges of modernising their armed forces within limited budgets. Concerns about China’s assertiveness in the South China Sea have further driven them to seek closer ties with powers such as Australia, Japan, and the United States. On the other hand, both the United States and China view Southeast Asia as a crucial strategic area, with Washington prioritising military interoperability and Beijing seeking regional influence through defence diplomacy and cooperation.
Three case studies of Malaysia, Indonesia, and Thailand reveal the nuanced approaches Southeast Asian nations take in their defence partnerships. Malaysia, for example, has a multifaceted strategy that includes ties to China while balancing its traditional relationships with Western powers such as the United States and Australia. Indonesia, as a non-aligned actor, engages with China, Russia, and the United States, prioritising military modernisation and technology transfer. Thailand, a United States treaty ally, has nonetheless deepened its defence ties with China.
Overall, the defence cooperation trends in Southeast Asia reflect a region striving for autonomy and resilience in the face of intensifying global strategic competition. Despite the Trump administration’s tariff policies, United States engagement with Southeast Asia on defence has been characterised by continuity rather than change. The increasing influence of China and the growing importance of middle powers such as Australia, Japan, and South Korea have reshaped regional defence dynamics. If these trends persist, Southeast Asia may split into maritime states with close defence ties to the United States and its allies, and mainland states with fewer such arrangements, potentially favouring China and Russia. The study concludes that if external partners fail to engage smaller nations such as Laos and Cambodia in defence initiatives, the geopolitical divide within ASEAN will deepen.