This chapter surveys the intertwining of religion (both Buddhism and Shinto) and war in the history of Japan, including: Buddhism’s initial introduction and subservience to the state in the 6th century; the emergence of warrior-monks (J. sōhei) in the Heian era; and the role of Buddhism in the samurai-dominated medieval period. Of particular interest to Western readers will be the emphasis on samādhi-power, acquired through Zen meditation, which was cultivated among the samurai class prior to the Meiji Restoration of 1868, contributing to the fanaticism that emerged in the modern Japanese Empire.