Trilateral cooperation has historically focused on managing security threats from North Korea and emerging tensions with China, while ensuring that Japan and South Korea – two of the US’s strongest allies – can deepen defence and security ties to preserve the balance of power in the region in their favour.
Close cooperation between the three is critical for maintaining deterrence against North Korea. Kim Jong Un’s regime poses a grave threat to international peace and stability. It continues to develop both its nuclear arsenal and its long-range missile systems, the latter of which could strike countries as far as 3,500 miles away. North Korea has a history of using brinkmanship, especially towards Japan and South Korea. It has also recently strengthened military cooperation and signed a mutual defence treaty with Russia, assisting Russia in its war on Ukraine, and threatening European security.
Deeper US–Japan–South Korea trilateral cooperation would allow the three countries to share military resources and intelligence assessments, and enhance joint capabilities in response to North Korean provocations. While the US–South Korea alliance is focused entirely on securing South Korea against the Kim regime in the North, the US–Japan alliance is aimed at maintaining peace in the Indo-Pacific region more broadly. The coordination of these two alliances in a trilateral format is crucial to check and respond to threats from North Korea, as Pyongyang threatens the security of all three countries at the same time.
Closer defence and security cooperation between the US, Japan and South Korea would send strategic signals to China of the West’s resolve to uphold the status quo in the Indo-Pacific.
Trilateral cooperation is also a vital element of US-led efforts to preserve the regional balance of power against a more assertive China. The rise of China’s economic and military capability has long-term implications for regional and global security. Beijing’s increasing territorial assertiveness in the Yellow Sea and East China Sea directly affects the sovereignty of South Korea and Japan, respectively. Meanwhile, China’s provocative actions across the Taiwan Strait and in the South China Sea, its use of unfair economic practices domestically and globally, its employment of cyber espionage, and active undermining of the rules-based international order are also seen as threats to regional and global security. Closer defence and security cooperation between the US, Japan and South Korea would send strategic signals to China of the West’s resolve to uphold the status quo in the Indo-Pacific. Alongside the military aspect, open cooperation on economics and technology is needed to compete with China’s growing heft in those areas. Through strengthened alliances and minilateral cooperation, Washington and its allies can match the scale of Chinese power.
China, North Korea and Russia are enhancing their own economic and military cooperation – as seen at the September 2025 ‘Victory Parade’ in Beijing attended by Kim and Russian president Vladimir Putin – and the nature of the threat to US and allied interests is becoming more sophisticated, with transfers of missile technology, defence equipment and even troops.
Coalitions such as the US–Japan–South Korea trilateral can build synergies and institutional arrangements between allies that can surpass the scale of cooperation among adversaries and thereby maintain deterrence. These coalitions can provide other countries in the region, such as those in Southeast Asia and the Pacific Islands, resources for their own economic growth and development, helping them become more resilient against external threats from state and non-state actors.
Minilateral partnerships also make it easier for states to solve collective action problems, such as climate change, global health and terrorism, overcoming a lack of capacity and efficacy of other multilateral arrangements in the Indo-Pacific and worldwide. For instance, during the COVID-19 pandemic, the Quadrilateral Security Partnership (Quad) of the US, Australia, India and Japan delivered vaccines to nations throughout the Indo-Pacific region.
By deepening security and defence ties, the US, Japan and South Korea can enhance interoperability, increase coordination among their militaries and even draw plans for coordinated responses in case of active threats to the regional status quo. In doing so, they will raise the costs of any belligerent action by Beijing or Pyongyang, and potentially increase the chance that the regional balance of power holds. The consequences of any such action by China or North Korea in the Indo-Pacific – whether in the Taiwan Strait or the South and East China Seas – would destabilize the world. US–Japan–South Korea trilateral cooperation is vital to ensure that such a situation does not come to pass.
