Lieber Studies Indo-Pacific Volume – A Tale of Tenacity, Tensions, and Ambiguity: Mutual Defense Treaties and Defense Groupings in the Indo-Pacific

Editors’ note: This post is based on the author’s chapter in Armed Conflict and International Law in the Indo-Pacific Region (Douglas Guilfoyle & Jennifer Maddocks eds. 2026), the thirteenth volume of the Lieber Studies Series published with Oxford University Press.

Five mutual defense arrangements concluded between 1951 and 1971 still anchor the security architecture of the Indo-Pacific. These are: the 1951 (at the time) trilateral ANZUS Treaty between Australia, New Zealand, and the United States; the bilateral mutual defense treaties between the United States and the Philippines (1951), Japan (1951, replaced in 1960), and the Republic of Korea (1953); and the 1971 Five Power Defence Arrangements (FPDA) between Australia, Malaysia, New Zealand, Singapore, and the United Kingdom.

The State Parties forged these arrangements during the Cold War, as well as in the shadow of the Korean and Vietnam Wars and the spread of communism through the region. All five remain in force today with striking durability. However, unresolved questions of treaty interpretation that trail behind them have proved durable as well. These questions have become more pressing as territorial disputes in the South and East China Sea intensify and as new security configurations such as AUKUS (including Australia, the United States and the United Kingdom) layer themselves onto existing frameworks.

This post draws attention to key issues identified in the chapter entitled “Mutual Defense Treaties and Defense Groupings in the Indo-Pacific Region: An Introduction,” from a recent volume in the Lieber Studies series titled, Armed Conflict and International Law in the Indo-Pacific Region. These issues include ambiguities surrounding the nature of assistance obligations under the respective treaty frameworks, as well as their geographic scope. The post also highlights challenges to the durability of these agreements (illustrated most vividly by the reconfiguration of the ANZUS in 1986), and the risk of competing or overlapping legal obligations as alliances proliferate.

Ambiguous Obligations of Assistance

None of the five defense arrangements obliges a partner to come to the military defense of another. Instead, each pairs an obligation of self-help and mutual aid to build defensive capacity with an unspecified commitment to “act” in the event of an armed attack on a partner. The ANZUS Treaty, for example, provides that the parties “recognise that an armed attack in the Pacific area on any of the parties would be dangerous to its own peace and safety” and declare that each State “would act to meet the common danger in accordance with its constitutional processes” (art. IV). The United States–Philippines Treaty of 1951, the United States–Japan Mutual Cooperation and Security Treaty of 1960, and the United States–Republic of Korea Mutual Defense Treaty of 1953 use very similar language. The FPDA Communiqué of 1971 goes no further, providing only that, in the event of an armed attack on Malaysia or Singapore, the five governments “would immediately consult together for the purposes of deciding what measures should be taken jointly or separately.”

The respective legal obligation to provide assistance therefore essentially amounts to a duty to consult and to consider a response, conditioned on each party’s domestic constitutional processes. That feature has nourished the persistent “fear of abandonment” that characterizes virtually every ally of the United States in the region.

Ambiguities Concerning Territorial Scope and Self-Defense

What the texts do not resolve in terms of commitment, they at times likewise leave ambiguous in terms of geographic application. This is no small matter when key controversies in the region include disputes over land features in the Indo-Pacific region whose sovereign status is contested. The Mutual Defense Treaty with the Philippines applies, by Article V, to attacks on “the metropolitan territory” of the parties, on “the island territories under [their] jurisdiction in the Pacific,” or on their armed forces, public vessels, or aircraft in the Pacific. Whether the formula extends to land features in the South China Sea is unsettled.

The 2016 Permanent Court of Arbitration award in Philippines v. China determined that China’s claim of historic rights within its “nine dash line” was without legal basis and the Philippines was entitled to a normal two-hundred-nautical-mile exclusive economic zone in the South China Sea. However, the tribunal expressly declined to rule on territorial sovereignty. The arbitration award consequently did not shed light on the issue of whether the Philippines can claim title over the disputed land features and whether this constitutes “jurisdiction,” in which case the Philippines could potentially argue that the 1951 Mutual Defense Treaty would apply to those land features. Successive United States administrations have affirmed that the treaty covers Philippine aircraft and public vessels in the South China Sea but have stopped short of confirming its application to any specific land feature.

The position is clearer under the 1960 United States–Japan Mutual Cooperation and Security Treaty. Article V applies to armed attacks “against either Party in the territories under the administration of Japan.” Read with the 1971 Okinawa Reversion Treaty (which returned administration of the Ryukyu and Daito Islands to Japan and extended the security treaty to them), the United States has long understood that formulation to cover the Senkaku/Diaoyu Islands, even though the United States has never taken a position on the underlying question of sovereignty. Even so, several administrations have consistently maintained that Japan’s administrative control over the islands should not be altered unilaterally.

Yet, two interpretive disagreements remain. The first concerns the meaning of “armed attack.” Japan’s reading is shaped by Article 9 of its 1946 Constitution, which renounces war and the threat or use of force as a means of settling international disputes. For decades Tokyo interpreted Article 9 as confining Japan to individual self-defense and as precluding the exercise of collective self-defense. Since 2014 an executive reinterpretation acknowledges a measure of collective self-defense where an attack on a State in a close relationship with Japan threatens Japan’s survival. Even so, Japan’s overall approach remains cautious, for instance, in its reluctance to treat the threat of a missile launch as itself an armed attack triggering self-defense.

The second disagreement concerns prior consent for the purpose of invoking collective self-defense. Article V is silent on whether the United States may invoke collective self-defense under the treaty absent a Japanese determination that an armed attack has occurred and a Japanese request for assistance. Japan has consistently maintained that any exercise of collective self-defense by the United States under the treaty hinges on Japan’s own exercise of individual self-defense, in consultation with the United States, a position that comports with the customary requirements articulated by the ICJ in Nicaragua v. the United States. Whereas Japan fears being drawn into a proxy conflict with China or Russia, the United States fears a restrictive reading that might preclude a preemptive strike in case of a missile threat from, for example, North Korea.

The 1953 United States–Republic of Korea Treaty raises a different interpretative challenge. Article III extends to attacks in the Pacific area on territories “now under their respective administrative control, or hereafter recognized by one of the Parties as lawfully brought under the administrative control of the other.” This reference to territories either already administered or that may in future come under administrative control, suggests its potential applicability to two ongoing disputes pertaining to land features which have thus far remained under the radar. The first concerns the Socotra/Ieodo Rock, an underwater feature in the overlapping South Korea–China Exclusive Economic Zones (EEZs) on which South Korea has constructed a tower supporting oceanic research. The second pertains to the Dokdo/Takeshima islands in the Sea of Japan and which are claimed by both Japan and South Korea. This latter dispute is particularly delicate because one United States bilateral alliance could, in extremis (if the dispute between South Korea and Japan were to escalate), be triggered against another.

Stress Tests for Durability

All five alliances were designed to last indefinitely, typically allowing for withdrawal with one year’s notice. However, none of the respective treaty frameworks contemplated suspension by a party. This gap was exposed in 1986 when the United States (and subsequently Australia) unilaterally “suspended” the ANZUS Treaty due to the refusal of New Zealand to allow nuclear-armed or nuclear-powered United States warships to enter its ports. Although constructs such as material breach under Article 60 of the Vienna Convention on the Law of Treaties (VCLT) or fundamental change of circumstances under Article 62 VCLT could potentially have been invoked, the United States never articulated a legal basis for the suspension. The ANZUS Treaty then effectively continued as a bilateral treaty between the United States and Australia, while defense cooperation with New Zealand continued on a different bilateral footing.

The only other alliance which has thus far been plagued by operational volatility is the United States–Philippines alliance. It was effectively suspended after the expiration of the Military Bases Agreement in 1991, but resumed again with the adoption of the 1998 Visiting Forces Agreement and reinforced by the 2014 Enhanced Defense Cooperation Agreement. Despite renewed termination threats expressed by then-President Duterte between 2016 and 2020, the alliance perseveres.

Competing Alliances and the Spectre of Conflicting Obligations

A third theme concerns the proliferation of overlapping security arrangements, with AUKUS being the clearest case. It is not, legally speaking, a mutual defense alliance as it does not pledge military assistance in the event of an armed attack. Instead, it is a capability and technology-sharing arrangement, most prominently covering nuclear-powered submarine technology. While direct normative conflict between AUKUS and ANZUS obligations is unlikely, the diversion of skilled personnel, infrastructure, and financial resources to AUKUS priorities is almost certain to constrain ANZUS-specific activity in the short to medium term. The same dynamic affects the FPDA, in which Australia and the United Kingdom are also AUKUS members.

The bilateral alliances face analogous pressures. Singapore’s long-standing security relationship with the United States and Malaysia’s 2014 comprehensive partnership with the United States may strengthen each State’s defense posture while drawing resources from FPDA exercises. Similarly, the 2022 United States-Japan-South Korea trilateral missile-warning arrangements, deepening Australia-Japan defense cooperation, and the April 2024 quadrilateral naval exercises in the Philippine EEZ generate networks of expectation that can in future complicate the legal interpretation and political operation of the older alliances.

Conclusion

Ultimately, several features common to the five defense arrangements analyzed stand out. First, they continue to demonstrate remarkable resilience in the face of adversity, absorbing setbacks ranging from New Zealand’s suspension from ANZUS to the temporary termination of Philippine base arrangements. Additionally, they have evolved into vehicles for security collaboration that extend well beyond collective self-defense against armed attacks, for which they were designed. The invocation of ANZUS after the September 11, 2001, attacks, the only invocation of any of the five alliances, illustrates how subsequent State practice has evolved from the original negotiating context.

Finally, precisely because none of the treaties obligates a forcible response and their territorial scope was deliberately or inadvertently left ambiguous at the time of adoption, the alliances now sit uncomfortably astride some of the most volatile maritime disputes in international relations. Questions such as whether the United States-Philippines treaty applies to features in the Spratlys, how Article V of the United States-Japan treaty interacts with Japanese constitutional constraints and the customary requirements for collective self-defense, whether the United States-Korea treaty could be triggered with respect to features Japan also claims, and how the proliferating layers of partnerships affect treaty performance, are not purely academic. They form part of the legal and political terrain on which a major pending regional crisis may turn.

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Erika de Wet is Professor of International Law and Head of the Department of International Law and International Relations at the University of Graz.

The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.

Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.

Photo credit: U.S. Navy, Petty Officer 2nd Class Karri Langerman