The “One Theater” concept that was unveiled at the recent U.S.-Japan defense ministers’ meeting is a proposal to integrate command and control (C2) in the East China Sea, South China Sea, and the Korean Peninsula into a single operational theater. This strategy seeks to maximize efficiency in countering China by consolidating the U.S. military’s dispersed forces. However, the origin of this concept lies not in the United States, but in Japan. Japan has been advocating for a “Japan-led security architecture” that treats the entire Indo-Pacific as one theater of operations, a vision welcomed by the second Trump administration. The U.S. Department of Defense is reviewing a plan to merge the East China Sea and South China Sea into a single theater and is pushing to establish a joint “S-QUAD (Security Quad)” operations coordination center with Japan, Australia, and the Philippines.

Japan’s intention is not simply to improve combat effectiveness. By integrating the Japan Self-Defense Forces (JSDF) with United States Forces Japan (USFJ), it aims to expand its C2 authority and access to intelligence while positioning itself as the hub for regional military operations encompassing the Korean Peninsula, the Taiwan Strait, and the South China Sea. The “One Theater” concept is a strategic idea to institutionalize a Japan-led Indo-Pacific order, which would in practice elevate Japan’s status as an equal operational partner to the U.S. military.

The South Korean government opposes this proposal, which was made without it being consulted in advance. The government has stated that “including the Korean Peninsula in Japan’s war planning framework is inappropriate.” The reasons for this are clear. First, if the current six-theater system is reorganized into a single, China-focused theater, it could dilute United States Forces Korea’s (USFK) mission to defend the Korean Peninsula. Second, if a U.S.-Japan–centered command structure is established, the mission of the ROK-U.S. Combined Forces Command (CFC) for the defense of the Korean Peninsula and South Korea’s decision-making authority could be undermined. Third, strengthening Japan’s operational role amid unresolved historical issues could provoke public backlash and sovereignty disputes within South Korea.

Given that the United States has already begun reviewing the idea of one theater, opposition alone is an insufficient response. South Korea needs to step forward as a strategic architect with its own proposal for a “Korean Peninsula–centered one East Asian theater.” North Korea’s provocations and China’s pressure on Taiwan are fundamentally linked, and crises could occur simultaneously in both theaters. Therefore, USFK should assume the central command role within this theater. Establishing an integrated system of ROK-U.S.-Japan intelligence sharing and joint training based on this framework would strengthen South Korea’s influence and position within the theater.

At the same time, discussions on redeploying U.S. tactical nuclear weapons should proceed concurrently. China and North Korea already field tactical nuclear weapons at combat-ready levels, but Japan’s pacifist constitution and Taiwan’s lack of a U.S. military presence make it difficult for them to secure credible nuclear deterrence. Ultimately, the only viable base capable of fielding tactical nuclear weapons is South Korea. If tactical nuclear weapons are redeployed to South Korea, they will provide immediate deterrence within the one theater and serve as justification for South Korea to cement its place as East Asian security’s key state.

The nature of the ROK-U.S. alliance must evolve from “defense of the Korean Peninsula” to an “alliance contributing to regional stability.” This is not simply an expansion of military roles but a strategic transformation in which South Korea becomes a key player in designing the security architecture of the Korean Peninsula and the region. The “One Theater” concept has already emerged as a new axis of Indo-Pacific security discourse. It is now up to South Korea whether it chooses to be led by Japan’s vision or takes the lead in reconfiguring ROK-U.S.-Japan trilateral cooperation. It is time for South Korea to step up as the architect, and the “East Asia–style One Theater,” centered on the Korean Peninsula, is the starting point.

* The view expressed herein was published on November 10 in The Munhwa Ilbo and does not necessarily reflect the views of The Asan Institute for Policy Studies.