Koshamain's War (コシャマインの戦い, Koshamain no tatakai) was an armed struggle between the Ainu and Wajin that took place on the Oshima Peninsula of southern Hokkaidō, Japan, in 1457. Escalating out of a dispute over a sword, Koshamain and his followers sacked twelve forts in southern Ezo (道南十二館), before being overcome by superior forces under Takeda Nobuhiro. The principal record of the conflict is Shinra no Kiroku.