Implications of U.S.-Japan Alliance Shifts and the “One Theater” Concept: Stronger Joint Response to North Korea in a Korean Peninsula Crisis Likely

Jang Hye-jin,

Research Fellow, Korea Institute for Defense Analyses

At the March 30 U.S.-Japan defense ministers’ meeting, Japanese Defense Minister Nakatani Gen proposed the “One Theater” concept to U.S. Secretary of Defense Pete Hegseth, drawing attention to its rationale and implications.

Japanese media report that this idea—combining the East China Sea, South China Sea, and the Korean Peninsula into one unified operational theater—originated from a senior officer in the Japan Self-Defense Forces (JSDF).

However, this concept has only been reported by The Asahi Shimbun, and even within Japan’s Ministry of Defense, there seems to be no consensus on its precise meaning. In other words, “One Theater” is still at a very preliminary stage and hasn’t been widely debated domestically.

Some even question whether the Japanese government is truly committed to developing the concept. It's also uncertain whether Japan’s efforts to expand JSDF’s regional security role and deepen defense cooperation with countries other than the U.S. represent a meaningful departure from past policies.

While much of the “One Theater” discussion centers on U.S.-China strategic dynamics, it may be more appropriate to view it as a reflection of shifts in Japan’s defense posture and the evolving structure of the U.S.-Japan alliance.

Under Article 6 of the Treaty of Mutual Cooperation and Security, the alliance extends to the “Far East,” not just Japan. The notion of “One Theater” likely stems from this regional framework. 

Japan has traditionally maintained a restrained, postwar defense strategy that restricted JSDF activities beyond its borders. Even with expanded cooperation under revised guidelines, its role largely remained confined to logistical support for the U.S., not direct involvement in regional security.

For much of the post-Cold War period, Japan saw itself as a rear-area actor, separated by time and space from the frontlines. Accordingly, the “One Theater” idea didn’t take hold. U.S. forces stationed in Japan were also primarily configured for support roles rather than direct combat.

However, evolving threats—such as North Korea’s growing missile capabilities and China’s assertive military expansion—have blurred the traditional distinction between front and rear. 

As North Korean missiles have gained range, Japan has come within striking distance, making it more likely to be targeted in any Korean Peninsula contingency. The increase in missile quantity and platform diversity has also challenged Japan’s missile defense systems, prompting the country to adopt a counterstrike capability. 

Meanwhile, China's expanding military and its aggressive maritime ambitions, particularly in areas like the East China Sea and around the Senkaku Islands, have posed an even more immediate threat. 

These factors influenced Japan’s decision to pursue collective self-defense. Previously, both counterstrike capabilities and the right to collective self-defense were constrained under Japan’s strictly defensive policy, seen as exceeding the minimum necessary force allowed.

Alongside this, growing strategic competition between the U.S. and China has heightened concerns about the possibility of conflict in the Taiwan Strait. 

For Japan, Taiwan is geographically close and security developments there are deeply linked to its own, especially given the presence of U.S. bases and territorial disputes over the Senkaku Islands. 

These concerns deepened after Russia’s invasion of Ukraine. Statements by former Prime Minister Abe—calling a Taiwan crisis a Japan crisis—and similar warnings from Prime Minister Kishida reflect Japan’s view of regional instability as a direct national security threat.

As Japan increasingly sees itself as part of the frontline rather than a safe rear area, it has begun shifting away from its traditionally passive defense posture. The focus now is on building real combat capabilities and readiness. 

Japan’s 2022 National Defense Strategy outlines the acquisition and use of standoff missiles for counterstrikes, emphasizes the need for transport capabilities to protect civilians, and highlights measures needed to sustain operations in times of conflict. 

To address evolving battlefield conditions and strengthen coordination with the U.S., Japan established the Joint Operations Command on March 24, 2025. Unlike the advisory-focused Joint Staff Office, it serves as a unified command for land, sea, and air JSDF forces.

The U.S. military presence in Japan is also being reshaped to enhance its combat role. Strengthening coordination between this new command and the U.S. Integrated Command helps transition the alliance toward a posture capable of joint combat. In this light, the “One Theater” proposal may carry implications that go beyond traditional security and diplomatic policy.

It is premature to determine exactly how changes in the U.S.-Japan alliance and the “One Theater” concept are interconnected. The newly established U.S. Integrated Command in Japan is still in its early stages, and as previously noted, the “One Theater” concept has yet to be clearly conceptualized. 

Likewise, it remains uncertain how Japan’s regional security role will evolve within the “One Theater” framework. While Japan has expanded its role in Korean Peninsula and Taiwan Strait contingencies beyond providing bases and rear-area support for the U.S. in response to changing security dynamics, its activities remain constrained outside Japanese territory and international waters. 

Counterstrike capabilities are exercised from within Japanese territory toward foreign targets, and collective self-defense is to be carried out in international waters. The Nansei Islands are also within Japanese territory.

If the “One Theater” concept strengthens U.S.-Japan coordination, their joint response to a Korean Peninsula crisis would likely improve, enhancing their regional influence—particularly Japan’s ability to support counterstrikes against North Korea.

One of the core missions of the new Joint Operations Command is to manage Japan’s standoff missile systems and its integrated air and missile defenses, including counterstrike capabilities. While Japan has maintained that any decision to launch such capabilities would be made independently, its limited satellite surveillance capabilities make joint operations with the U.S. a more viable and immediate option.

Strengthening coordination between Japan’s Joint Operations Command and the U.S. Integrated Command under the “One Theater” framework could make counterstrikes against North Korea more viable, potentially under collective self-defense. Continued attention is needed to see how this evolves and influences Japan’s response to a Korean Peninsula crisis.