On October 12, 2025, a Chinese Coast Guard vessel fired water cannons at and collided with the BRP Datu Pagbuaya, a Philippine government fisheries vessel, near Thitu Island in the South China Sea. Within hours, the United States condemned the incident and reaffirmed its support for Manila, its treaty ally in Asia.
Behind the Philippines stands a treaty signed in 1951 that obliges the United States to act if Philippine ships, aircraft, or armed forces come under attack.
Such agreements are rare in Southeast Asia. Of the region's eleven countries, only four are bound by formal defense alliances with external powers. The rest maintain defense cooperation arrangements, joint military exercises, and strategic partnerships, but without any commitment to come to one another's defense.
Formal Alliances Commit States to Fight
A formal defense alliance is a written treaty that obliges one country to assist another if it is attacked. This is what distinguishes it from ordinary defense cooperation.
The Manila Pact, which covered the Philippines and Thailand, states that an armed attack within the treaty area constitutes a threat to all members and must be addressed in accordance with each member state's constitutional processes.
Defense cooperation agreements, by contrast, contain no such obligation. Joint exercises, military training, arms transfers, and security dialogues can all take place without either side being legally required to defend the other.
Nearly every Southeast Asian country maintains this type of relationship, whether with the United States or with China. Vietnam, for example, was elevated to a Comprehensive Strategic Partnership with the United States in 2023, but that relationship is a partnership rather than an alliance.
The Philippines: The Strongest Alliance and Growing Stronger
The Mutual Defense Treaty between the Philippines and the United States was signed on August 30, 1951, and remains the cornerstone of Manila's defense policy today.
In February 2026, the two countries reaffirmed that the treaty covers attacks on their armed forces, aircraft, and public vessels, including coast guard ships, anywhere in the Pacific Ocean and the South China Sea.
The 12th Bilateral Strategic Dialogue in Manila during the same month marked the 75th anniversary of the alliance while also expanding military and economic cooperation.
In early June 2026, the defense ministers of both countries reiterated their commitment to the alliance on the sidelines of the Shangri-La Dialogue in Singapore.
Thailand: Bound on Paper, Looser in Practice
Thailand is tied to the United States through the same instrument as the Philippines: the Manila Pact. The difference is that the pact was not originally a bilateral agreement but the foundation of a regional defense organization known as the Southeast Asia Treaty Organization (SEATO).
SEATO was dissolved on June 30, 1977, after many of its members lost interest in the organization.
What preserved Thailand's status was Article X of the Manila Pact, which states that the treaty remains in force indefinitely. As a result, although the organization disappeared, the treaty itself remained legally valid.
This continues to serve as the basis of U.S. security commitments to Thailand today, reinforced by the Thanat–Rusk Communiqué and Thailand's designation as a Major Non-NATO Ally since 2003.
Yet while the alliance remains strong on paper, it has gradually loosened in practice. As of January 2026, amid intensifying U.S.-China competition, Bangkok has increasingly moved closer to Beijing. Washington reportedly believes Thailand would be unlikely to grant access to its military facilities in the event of a conflict in the Taiwan Strait.
Singapore and Malaysia: A Non-Binding Pact That Endures
Through the Five Power Defence Arrangements (FPDA), established in 1971, Singapore and Malaysia are linked with Australia, New Zealand, and the United Kingdom.
The key difference is that the FPDA is not a binding alliance. It does not guarantee automatic military assistance. Instead, it commits the five countries to consult one another if Singapore or Malaysia comes under attack.
Even so, the FPDA remains active. On May 29, 2025, the defense chiefs of the five member states issued the FPDA Exercise Concept Directive 2025 and approved a five-year roadmap for the arrangement's future development.
Seven Countries Choose Neutrality
Indonesia, Vietnam, Myanmar, Cambodia, Laos, Brunei, and Timor-Leste are not members of any formal defense alliance.
Indonesia, in particular, adheres to its bebas-aktif (independent and active) foreign policy doctrine, which rejects alignment with competing geopolitical blocs.
This pattern has deep historical roots. When SEATO was established in 1954, Indonesia and Burma chose neutrality, while Laos, Cambodia, and Vietnam were prohibited from joining military alliances under the Geneva Accords.
The result is a Southeast Asian security landscape that remains divided today. Maritime states such as the Philippines have moved closer to the United States, some mainland states have gravitated toward China, and many others continue to position themselves in between, benefiting from cooperation with all sides while avoiding formal commitments to any of them.
