蒙古襲来と刀の革命

In October 1274 (Bun'ei 11), a combined Mongol-Goryeo fleet landed on the shores of Hakata Bay in northern Kyushu — the first full-scale foreign invasion Japan had ever experienced — delivering a shocking combat experience to Japanese warriors. A second, larger expedition in 1281 (Kōan 4) was repelled when both fleets were destroyed by storms that became legendary as kamikaze (divine winds). Mongol tactics were wholly unlike anything Japanese samurai had encountered: rather than the ideal of individual challenges and single combat, Mongol forces attacked with coordinated mass formations combining bows, polearms, and explosive projectiles (tetsuhau). Japanese swords revealed a critical weakness in this combat environment. Mongol soldiers wore leather lamellar armor (hikou) that deflected tachi cuts unexpectedly and reduced the effectiveness of thrusting attacks. The practical demands of fighting through this armor — and responding to massed formations rather than individual opponents — fundamentally changed what was required of sword design. Before the invasions, the ideal tachi had been an elegant blade with deep koshi-zori, a slender profile, and a small ko-kissaki. After the invasions, warriors demanded broader blades with greater kasane and extended kissaki — a form now termed genkou-go-taihai (post-Mongol-invasion proportions). Smiths of the Osafune school in Bizen, including Nagamitsu, Kagemitsu, Kanemitsu, and Chōgi, left behind works that document these transformations. These late-Kamakura tachi stand as living historical witnesses to the way Japanese swords evolved in direct response to battlefield experience — a profound dimension of meaning that gives these pieces special significance for collectors.