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Paper short abstract

This paper analyzes Japan’s reinterpretation of collective self-defense through the concept of senshu-boei, with particular focus on “minimum necessary capability.” It examines the 2014 policy shift, implications for Taiwan, and Japan’s expanding denial-oriented security posture in the region.

Paper long abstract

Amid China’s rapid military expansion and increasingly assertive behavior in the South and East China Seas and the Taiwan Strait, Japan’s cabinet under the late Prime Minister Shinzo Abe approved the limited exercise of the right to collective self-defense in 2014, followed by security-related legislation in 2015. Existing scholarship largely attributes this shift to the cabinet’s reinterpretation of Article 9 of the Constitution, which renounces the use of force in international disputes. While this literature traces the post–Cold War evolution of Japanese security policy, particularly under Abe, it fails to explain the political, legal, and doctrinal conditions that enabled such reinterpretation.

This study addresses that gap by examining Japan’s evolving interpretation of collective self-defense through the framework of senshu-boei (専守防衛, exclusive self-defense), focusing on its core principle: hituyō saishōgen no nōryoku (必要最小限の能力, minimum necessary capability). How this standard is defined and reinterpreted is central to understanding Japan’s postwar military trajectory and future transformation. The paper also analyzes the implications of the 2014 reinterpretation for Taiwan’s security, where collective self-defense scenarios are most plausibly relevant, and examines Japan’s growing reliance on denial-oriented, cross-domain capabilities.

This analysis is timely as cross-strait tensions have intensified. China’s sharp response to Japanese Prime Minister Sanae Takaichi’s 2025 statement that a Taiwan contingency would threaten Japan’s survival, thereby indicating that it could be regarded as meeting the criteria for exercising collective self-defense, underscores the issue’s sensitivity. Because Taiwan’s security is closely linked to Japan’s own, interpretations of the “minimum necessary capability” carry significant strategic consequences. The paper concludes that if further reinterpretation becomes untenable, Japan may be compelled to move beyond senshu-boei and adopt the Ashida interpretation of Article 9, which permits the full use of collective and individual self-defense.