President Donald Trump assumed office in January 2017 embracing a ‘transactional’ outlook ^[1] towards alliance politics. US alliances would be managed less on traditional friendship and more on assessments of how the United States would gain alliance affiliation. For America’s longstanding Southeast Asian regional treaty allies as Thailand and the Philippines, the implications of the new US administration’s transactional posture for future relations with Washington were unclear. Both of these allies had already moved far towards hedging against the US by substantially upgrading their economic—and, in Thailand’s case, military—ties with China.

More than halfway through his elected term of office, however, Trump has drifted back towards implementing a strategy for the Indo-Pacific that’s more in tune with those of previous US administrations. He announced his support for a ‘free and open Indo-Pacific’ during a November 2017 trip to Asia and confirmed it the following month with his release of a new national security strategy. The fundamental precepts ^[2] included the principles of freedom of navigation and overflight, the rule of law, freedom from coercion, respect for sovereignty, private enterprise and open markets, and freedom of independence of all nations.

As I explain in a new ASPI report ^[3], released today, the national security strategy’s obvious emphasis on maritime and commercial interests dovetailed naturally with a renewed US focus on the ‘southern flank’ component of its Indo-Pacific alliance network and on greater Southeast Asia. The southern-flank portion of the US regional alliance network—also known as the ‘San Francisco system’—includes formal bilateral defence treaties with Thailand and the Philippines and the ANZUS Treaty with Australia. It also involves substantial American strategic partnerships with such ASEAN states as Singapore, Malaysia, Indonesia and Vietnam that complement the more formal components of US alliance networking in the region.

ASEAN’s geographical location places it squarely in the centre of two key sea lanes of communication through which much of the world’s maritime commerce flows—the Malacca Strait and the South China Sea. If the US were denied access to—and the ability to control—such conduits during war or high crises, its influence within, and its ability to traverse and trade throughout, the entire Indo-Pacific region would be critically impeded. To preclude such outcomes, various Southeast Asian states must, combined with their ANZUS counterparts, work to sustain an enduring balance of power in the region.

Trump has worked visibly to modify the tensions with both the Philippines and Thailand that intensified under the Obama administration. He repeatedly praised ^[4] Rodrigo Duterte when attending the 2017 ASEAN summit which the Philippines leader hosted, downplayed ^[5] the human rights issue and emphasised continued US support for Philippine counterterrorism operations.

In March 2019, US Secretary of State Mike Pompeo visited the Philippines and outlined a significant shift in Washington’s traditional interpretation of the US–Philippines Mutual Defense Treaty by explicitly warning ^[6] that ‘[a]ny armed attack on Philippine forces, aircraft or public vessels in the South China Sea will trigger mutual defence obligations’.

Trump hosted Thailand’s Prime Minister Prayuth Chan-ocha at the White House in October 2017. The visit partially modified ^[7] Thai–American tensions over human rights and facilitated increased US–Thai security cooperation. Bilateral Thai–US military exercises were also accelerated ^[8].

In the broader Indo-Pacific, the US has, for well over a decade, cultivated upgraded defence ties with India, Indonesia, Malaysia, Singapore and Vietnam. Indonesia’s strong legacy of non-alignment, and its self-appointed role as ASEAN’s guardian against external great-power competition, still render Indonesia’s partnership with both Australia and the US, however, as qualified. Closer US–Malaysian defence ties are being developed as a counterweight to the ‘China factor’. For example, the Cope Taufan 18 bilateral tactical airlift exercise ^[9] held in July 2018 focused on air superiority, airborne command and control, interdiction, air refuelling, and tactical airlift and airdrop.

Despite its relatively small size and population (approximately 5.7 million), Singapore is viewed ^[10] by many in Washington and Canberra as a de facto ally with its own highly modernised military underpinned by a comparatively high defence spending rate and motivated by common threat perceptions, including fears of an unstable South China Sea. Australia and Singapore signed a comprehensive strategic partnership agreement in 2015.

The US–Vietnam Bilateral Defence Cooperation Memorandum of Understanding in 2011 initiated collaboration in maritime security. US assistance in developing the Vietnam Coast Guard’s participation in the Rim of the Pacific maritime exercise, and the lifting of a long-term American ban on arms sales to Vietnam in 2016, have been reciprocated by at least tacit Vietnamese support for US naval activities in the South China Sea. This positive momentum is tempered, however, by Vietnam’s still constrained posture that precludes alienating the more geographically proximate and economically crucial China relationship.

US regional allies and partners remain uncertain about American staying power in their neighbourhood. The slow development of the Australia–India–Japan–United States Quadrilateral Defence Initiative, the relatively modest US funding to promote Southeast Asia’s regional security (US$300 million ^[11]) and China’s determined efforts to cultivate enhanced regional influence have reinforced ASEAN’s propensity to apply hedging strategies designed to preclude either American or Chinese dominance over their region. But America’s continued presence is supported throughout Southeast Asia.

In these circumstances, applying the ANZUS alliance to breathe new life into the San Francisco system’s southern flank and its peripheries may well be worth pursuing. Three such measures immediately come to mind and are offered as policy recommendations here. One is upgrading joint policy planning between the ANZUS allies and Manila on how to respond forcefully and credibly to Chinese challenges of Philippines-claimed territories in the South China Sea. A second is for the US and Australia to extend their already well-established diplomatic posture of ceding the political destiny of Vietnam to the Vietnamese people to both the Philippines and Thailand. Finally, any Australian government, of whatever persuasion, must remain steadfast in encouraging US policy consistency in its alliance politics in the Indo-Pacific.

The durability of the San Francisco system remains one of the democratic community’s most valued security frameworks. ANZUS policymakers could do far worse than to assist in ensuring that it stays that way.