Japan’s Tomahawk and JSM Missile Acquisition and the Legal Questions Surrounding Collective Self-Defense

Japan has begun taking delivery of U.S.-made Tomahawk cruise missiles and Norwegian-developed Joint Strike Missiles (JSM), marking a significant shift in its defense capabilities. This acquisition, intended to bolster Japan’s counterstrike abilities, has reignited debate surrounding the legality of collective self-defense, particularly in the context of a potential Taiwan Strait crisis.

Expanding Counterstrike Capabilities

Japan’s Defense Minister, Shinjiro Koizumi, announced the commencement of deliveries on March 13, 2026. The Tomahawk missiles, with a range of approximately 1,600 kilometers, and the JSM, with a range of 500 kilometers, will be deployed to enhance Japan’s ability to strike targets from outside enemy range. The Tomahawk missiles will initially be deployed on the Maritime Self-Defense Force destroyer, Chokai, currently undergoing remodeling at Sasebo Base. The JSM will be mounted on Japan’s F-35 fighter jets.

The Debate Over Collective Self-Defense and Taiwan

Alongside this military build-up, Japanese officials have increasingly discussed the possibility of invoking collective self-defense in a Taiwan Strait contingency, particularly if the United States is involved. Prime Minister Sanae Takaichi has suggested that such a scenario could constitute a “survival-threatening situation,” potentially justifying military intervention under Japan’s security legislation.

Legal Challenges to Japan’s Position

Yet, legal experts argue that Japan’s interpretation of collective self-defense is questionable under international law. The right to collective self-defense, as outlined in Article 51 of the United Nations Charter, is not a discretionary tool activated by alliance solidarity but is contingent upon a legally recognized victim state experiencing an armed attack.

Taiwan’s legal status complicates matters. The Cairo Declaration of 1943 and the Potsdam Proclamation of 1945 stipulated that Taiwan should be returned to China, a position Japan formally accepted. UN General Assembly Resolution 2758 further recognized the People’s Republic of China as the sole legitimate representative of China. Taiwan cannot be treated as a sovereign state capable of exercising individual self-defense or requesting collective defense assistance.

attempting to invoke collective self-defense to assist the U.S. If American forces were attacked in the Taiwan Strait would too be legally problematic. Collective self-defense must originate with the victim of an armed attack, not a third party. If the U.S. Were to intervene militarily without UN Security Council authorization, its legal standing would be contested, and Japanese support could be considered assistance in an internationally wrongful act.

Implications for Regional Stability

The acquisition of these advanced missile systems and the debate surrounding collective self-defense underscore Japan’s evolving security posture. While Japan maintains that these capabilities are solely for self-defense, the move has raised concerns about a potential arms race and increased tensions in the region. The restrictive design of the UN Charter, intended to prevent alliance-driven conflicts, remains a critical factor in maintaining regional and global stability.