US playing alliance cards in the South China Sea

By Choo Jae-woo

During U.S. National Security Advisor Jake Sullivan’s visit to Beijing last August, an important development went largely unnoticed by the media. Besides being the first such visit by a U.S. official in 11 years, it marked the first time the U.S. revealed its defense stance regarding the South China Sea. Sullivan conveyed to his counterpart, Chinese Foreign Minister Wang Yi, that the U.S. would defend an ally’s territorial disputes with China if they escalated to military conflict. Sullivan’s ally reference explicitly meant the Philippines, aligning this defense with freedom of navigation in the region. This declaration is significant, marking the first effective use of a U.S.-led alliance to secure stability in the region since the U.S.’ 1954 defense commitment to Taiwan, which extended protection to the Jinmen and Mazhu Islands, key gateways to the South China Sea.

A significant aspect of Sullivan’s visit was his meeting with Zhang Youxia, vice chairman of China’s Central Military Commission, making it the first meeting in eight years between America’s top security official and China’s second-ranking military officer in Beijing.

Sullivan aimed to restore military dialogues between the U.S. Indo-Pacific Command and China’s People’s Liberation Army (PLA) Southern Theatre Command. This resulted in the two sides holding a video conference on Sept. 9, during which the U.S. urged the PLA to reconsider its aggressive tactics in the South China Sea. The meeting implied a U.S. defense scope extending to the Taiwan Strait and East China Sea, areas with existing territorial disputes involving Japan and China.

For South Korea, Sullivan’s stance on defending the Philippines offers a strategic lesson. The focus here is on the U.S.’s support of an ally amid territorial disputes with China in the South China Sea, reflecting the possibility of involvement in the Taiwan Strait due to the Philippines’ geographic proximity. Sullivan based this on the U.S.-Philippines Mutual Defense Treaty, which states that if the territorial integrity or security of either country is threatened in the Pacific, both must consult and respond per their constitutional procedures. This was the first explicit invocation of mutual defense since President Obama’s 2014 declaration of support for Japan’s territory under the U.S.-Japan Mutual Defense Treaty (Article 5), due to the escalating Diaoyu (Senkaku) Islands dispute.

Sullivan’s move hints at a shift in U.S. strategy in the South China Sea, the Taiwan Strait and the broader First Island Chain. It suggests that the U.S. might extend the scope of alliances, like the U.S.-ROK alliance, to broader regional security issues. The U.S.-ROK alliance remains an exception in the Indo-Pacific, but adjustments are looming.

The strategy leverages the obligations embedded in U.S. alliances, which include provisions for joint responses to threats in the Pacific. Previously, these obligations were largely dormant due to the relatively weak military capacities of regional nations and the lesser threat posed by China. Despite China becoming a nuclear power in the 1960s, its conventional military capacity was not enough to challenge U.S. interests directly.

However, China now wields considerable military power, significantly threatening regional peace, stability and U.S. strategic interests, particularly concerning freedom of navigation in these contested waters. The U.S. is now at a disadvantage; its military forces are outnumbered, making it difficult to unilaterally ensure regional stability. Therefore, the U.S. is now invoking the long-dormant Pacific clauses of its alliance treaties to ensure a collective defense response.

For instance, the U.S.-Philippines Mutual Defense Treaty stipulates that if either party’s territorial integrity or security is threatened in the Pacific, both must consult and respond together. The treaty covers attacks on not just the territories but also military forces, public vessels or aircraft. A similar provision exists in the 1951 ANZUS Treaty (with Australia and New Zealand), which promises consultations and joint responses to threats in the Pacific. Although the U.S.-Japan Mutual Defense Treaty of 1951 refers to the "Far East" rather than the Pacific, the Far East’s geographical scope back then was broad, including areas between the First and Second Island Chains.

Although the U.S.-Taiwan Defense Treaty ended when the U.S. established diplomatic ties with China, the U.S. commitment to Taiwan’s defense remains based on the principles of the former treaty. The 1954 treaty identified the Western Pacific as the region of defense (Article 5), with the U.S. viewing its obligations as extending to islands under its jurisdiction, such as Guam and Saipan, within the Second Island Chain (Article 6). This implied a shared defense responsibility for these strategic areas in the Western Pacific.

South Korea is not exempt from these provisions. The 1953 U.S.-ROK Mutual Defense Treaty also includes a clause (Article 3) for joint responses to threats in the Pacific region. The U.S. is now urging South Korea to clarify its stance in the event of a crisis concerning Taiwan. With most U.S. allies in the region having already stated their positions, the U.S. aims to determine how to align the U.S.-ROK alliance with regional developments. This signals a shift in the primary focus of the U.S.-ROK alliance away from deterring North Korea alone. The establishment of the Nuclear Consultative Group is part of this effort to redefine the role and function of the U.S.-ROK alliance and U.S. Forces Korea.

As geopolitical dynamics evolve, regardless of the U.S. administration in power, South Korea will need to clarify its stance on the U.S.-ROK alliance’s role in the Pacific, including Taiwan. This will test South Korea’s pursuit of its global pivotal state diplomacy. In the meantime, the U.S. is broadening its alliances to include collective responses to threats across the Pacific, focusing on containing China’s growing military power and securing stability in critical maritime regions.

Choo Jae-woo is a professor at Kyung Hee University.