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 the purchase of 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.
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