South Korea is the ideal anchor for the first island chain
Overwhelmingly, commentary regarding Chinese military aggression focuses on the area known as the “first island chain,” stretching from the Philippines to the south, through Japan in the north, with Taiwan at the centerpiece. Taiwan remains the focus because control of Taiwan, whether by military force or other coercive means, remains a national security objective of the Chinese Communist Party. As this geography includes mutual-defense treaty allies of the United States, it is understood as a multinational chain restraining Chinese hostility. Should China attempt a military seizure of Taiwan, the likelihood of the conflict expanding to include the entirety of the first island chain—and US forces stationed or deployed there—remains high.
However, this discussion largely omits another likely participant that could prove decisive in deterring such a fight and in determining its outcome: South Korea. Indeed, while an island only in the sense that its infrastructure is not connected to the Asian mainland because of the obstacle of North Korea, South Korea is geographically the “anchor” of the first island chain and could operationally serve in that role. By addressing the assumptions that lead analysts to exclude Korea from assessments of Chinese military aggression, the advantages to regional security that arise from Korean investment become clear.
Why is Korea excluded from discussion of a US-China conflict involving Taiwan? The United States currently assesses China as the country’s pacing threat, and a Chinese invasion of Taiwan could likely be a catalyst for war between the United States and China. South Korea’s primary threat—North Korea—is not perceived to be as dangerous to US national security interests relative to China. US defense planners compartmentalize the threats posed by China and North Korea, and there is an incorrect perception that US military assets stationed in Korea cannot be used in a fight against China. As a result, Korea gets short shrift in this discussion. But the assumption that Korea would be excluded from such a conflict is baseless.
First, this bias is premised on the assumption that South Korea would only fight alongside the United States in a war against North Korea and not become involved in a war between the United States and China. However, the US-South Korean mutual defense treaty does not contain this limitation. Rather, it states clearly: “Each Party recognizes that an armed attack in the Pacific area on either of the Parties . . . declares that it would act to meet the common danger in accordance with its constitutional processes.” Moreover, attitudes within South Korea are becoming more supportive of military assistance to Taiwan in the event of a Chinese attack. An attack against the United States in the Pacific is increasingly likely to facilitate South Korean participation in that conflict, regardless of the belligerent. And in a US-China war over Taiwan, China has many sound operational reasons for attacking US forces across the region and in Korea.
Second, analysts assume that US forces and matériel placed in South Korea is held hostage there, so to speak, and may only be used to defend South Korea against North Korea. Within US military planning circles, this is referred to as the “black hole theory” of the Korean peninsula. Indeed, Combined Forces Command, the warfighting headquarters that fully integrates US and South Korean forces, is optimized for and conducts exercises to defend South Korea from North Korean aggression.
However, the mission of Combined Forces Command is not exclusive to North Korean threats and includes deterring and defeating “outside aggression” against South Korea. Similarly, US Forces Korea, which is separate from Combined Forces Command, deters and defends against any aggression to maintain stability in Northeast Asia. The former commander of both organizations, General Paul LaCamera, publicly noted that planning for his forces encompasses any contingency, including those that occur outside of Korea. His successor and the current commander, General Xavier Brunson, reinforced this notion by highlighting the proximity of South Korea to China and the broader, regional, strategic utility of US forces in South Korea beyond the threat posed by North Korea.
Finally, there is no agreement, regulation, or document that traps US forces on the Korean peninsula. If the Pacific becomes a war zone, no matter the threat, the United States gets a decisive vote in how its forces are employed, whether on or off the Korean peninsula. US forces have historically been deployed far from Korea for combat operations elsewhere. In the most recent example, a Patriot missile unit deployed from Korea defended the Al Udeid airbase in Qatar from Iranian missile attacks in June of 2025. In short, the black hole theory is a myth. That it is a myth is illustrated perhaps most clearly by the other side of this coin—the North Korean threat. North Korea has shifted munitions and troops from the Korean peninsula in support of the Russian war of aggression in Ukraine. Troops and matériel will be committed wherever each nation’s security interests lie, regardless of their geography.
These facts should be contextualized against the real threat of simultaneity of conflict throughout the region. Great power wars tend to expand horizontally and there are many paths by which a Chinese invasion of Taiwan could lead to a North Korean attack on South Korea and vice versa. Further, the likelihood of Chinese interference in a Korean crisis will only increase as Chinese-US tensions rise. Finally, an increased footprint of US forces in Japan as a check against these threats is an untenable solution, as the US started honoring its legal obligation to start reducing total forces in Okinawa last year.
To summarize:
- the United States should expect South Korean co-belligerency if the United States is attacked in the Pacific region per the US-ROK mutual defense treaty;
- there is nothing preventing the use of US assets staged on the Korean peninsula in contingencies that occur off the Korean peninsula;
- any Chinese-initiated conflict will almost certainly expand to Korea in some fashion; and
- a North Korean-initiated conflict will almost certainly involve Chinese belligerency to some degree.
This being the case, opportunities exist for integrating South Korea into a wider defense across the first island chain.
First, investing US resources in South Korean security should rightly be seen as simultaneously defending against both North Korean and Chinese aggression. This form of dual deterrence is akin to moving a chess piece to simultaneously threaten two enemy pieces. The Chinese certainly view it this way, illustrating their perception quite clearly in their angry reaction to the 2016 deployment of a US Terminal High Altitude Anti Area Defense anti-missile battery in South Korea. While explicitly deployed to intercept North Korean missile strikes, this battery could also intercept Chinese missile launches, and this reality triggered a plethora of Chinese retaliatory and coercive actions against South Korea, to which South Korea did not yield.
Second, South Korea should be viewed through the lens of its proximity to the decisive space for any potential fight and not through the outdated view of compartmentalized conflicts. This immediately solves fundamental operational problems of time, space, and force. Forces stationed outside of the first island chain, especially those based in the continental United States, likely won’t be able to safely enter the conflict zone once it becomes contested in time to influence the decisive, opening phases of any conflict. Forces stationed within South Korea, meanwhile, are in position to support the defense against threats from both North Korea and China.
With these views informing policy, forces that could be useful in either a Korea or China scenario currently stationed in the United States could instead be based in South Korea. For example, US Army Pacific continues to advertise its potential as the linchpin of the joint force in a China fight, but it is hard pressed to fill this role because its forces are overwhelmingly based in the United States—over 5,000 miles away from the first island chain. Moving such mass across the Pacific Ocean without dominance of the air and maritime domains will only sacrifice an unacceptable number of US troops for no operational gain. But basing them instead in South Korea closes this distance, steals a march against two adversaries at once, allows for a much more rapid shift of these forces to key terrain at the start of a fight, and contributes meaningfully to deterrence before a fight.
Similarly, viewing South Korea as a more regionally focused power-projection platform allows other options for US forces departing from Okinawa, per the terms of the Defense Policy Review Initiative: The US government is obligated by the government of Japan to remove 9,000 US Marines from Okinawa, of the over 18,000 stationed there, a movement that has already begun. The US military is currently on track to move these troops to Guam and Hawaii. This places nearly one-half of this element of the US stand-in force beyond the first island chain, undermining a key element of US defense posture across that chain. But Korea is thousands of miles closer to any place US forces will fight. Moreover, Korea, where anti-Chinese attitudes are on the rise, would likely accept the additional investment. Korea would foot much of the bill for infrastructure supporting these additional forces per the terms of the Special Measures Agreement and is likely to continue to pay for their sustainment in the future.
Finally, the US military could store critical munitions and other material required for a fight against either North Korea or China in South Korea. The extensive lines of communication between the first island chain and resupply points in Guam, Hawaii, and the continental United States would impose great cost on any US campaign in the region. Again, South Korea’s proximity and available space make it ideal as an anchor for the first island chain. Moreover, South Korea has demonstrated its willingness to tap into its own wartime stocks, illustrated by its donation of more artillery shells to Ukraine than all of those donated by Europe combined. Similarly, South Korea won’t prevent the United States from using its munitions for any fight it chooses to, whether against North Korea or China.
The risk of simultaneous wars with China and North Korea only seems to increase, and such an occurrence would undermine regional and global stability. Neither of these fights are isolated. Should the United States defend Taiwan against Chinese aggression, South Korea will inevitably become involved. So, too, will China become involved in any US effort to defend South Korea from North Korean attack, likely seizing the opportunity to make coercive and military gains against Taiwan.
US planners should build these assumptions into their planning frameworks to seize the opportunities that exist. Peninsular stability and Taiwan Strait stability are one and the same. Investing in South Korean security serves as an investment in Taiwan security.
Lieutenant Colonel Brian Kerg is an active-duty US Marine Corps operational planner and a nonresident fellow in the Indo-Pacific Security Initiative at the Atlantic Council’s Scowcroft Center for Strategy and Security. He is also a 2025 nonresident fellow with the Irregular Warfare Initiative, a 501(c)3 partnered with Princeton’s Empirical Studies of Conflict Project and the Modern War Institute at West Point.
The views expressed here are those of the author and do not represent the positions or opinions of the US Marine Corps, the Department of Defense, or any part of the US government.
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Image: US Marine Corps photo by Lance Cpl. Jonathan Beauchamp