This chapter discusses origins and development of Japanese pacifism and renunciation of war. Despite the Constitution provides for the renunciation of war as an instrument to settle international disputes and a ban on the maintenance of any war potential, Japan has progressively established the SDF (Self-Defence Forces), allowed US military bases on national soil, participated to UN peace keeping operations and supported Ukraine in the present-day war. The Japanese approach to war and to self-defence has evolved significantly since 1946, following the so-called revision (of the Constitution) by interpretation performed by the government. According to most constitutional scholars, the whole Japanese defence architecture is unconstitutional. However, the Supreme Court, resorting to the political question doctrine, has always refused to discuss the issue. The ongoing war in Ukraine is again putting under strain the ambiguities of Japanese defence and pacifism.