In 2014, Japan’s cabinet approved a significant change to national security policy. Previously barred from using military force, except in cases of self-defence, a constitutional reinterpretation by the cabinet allowed “collective self-defence”—using force to defend itself and its allies. The decision was controversial, considering post-war pacifism is firmly entrenched in Japanese national identity. I analyse how national security has been portrayed in the policymaking process for reinterpreting the Constitution. Meanwhile, since the early 2010s, Japanese society has been rocked by demonstrations opposing this. I explore the rise of a new youth activist movement in response to the proposed legislation. In particular, I argue that new ideologies and strategies appealed to young people in the organising of various protests, focusing on how they interpret the national security discourse and locating these social movements in Japanese post-war peace activism.