Kōan Campaign — The Second Mongol Invasion and the Swords of Kamakura

The Kōan Campaign of 1281 was the second and far larger of the two Mongol invasions of Japan. Seven years after the first invasion (the Bun'ei Campaign of 1274), Kublai Khan dispatched a combined force of approximately 140,000 troops on some 4,400 vessels — the Eastern Route Army (40,000 troops, 900 ships) and the Jiangnan Army (100,000 troops, 3,500 ships) — toward Hakata Bay in northern Kyushu.

The Kamakura shogunate, under regent Hōjō Tokimune, had prepared thoroughly. Between 1276 and 1281, a stone wall (bōrui) roughly 20 kilometres long, 2–3 metres high, and 3 metres wide was constructed along the Hakata Bay shoreline, blocking the flat beaches used by the Mongol cavalry in 1274. The shogunate also institutionalized a permanent coastal watch duty (ikoku keigobanyaku) for Kyushu warriors.

The Eastern Route Army arrived first, landing on Iki and Tsushima in late May, then attacking Hakata Bay on June 6. The stone wall proved unexpectedly effective; Japanese warriors defended from behind it with bows, naginata, and swords, while launching repeated night raids in small boats against the anchored fleet. Unable to advance inland, the Eastern Army withdrew to Takashima to await the Jiangnan Army.

When the two fleets united in late June, they anchored in dense formation off Takashima — and on the night of July 30, a major typhoon struck the coast. The crowded anchorage became catastrophic: ships collided and sank by the hundreds. Survivors who reached shore were hunted down by Japanese warriors. Only a remnant escaped back to the continent.

The campaign confirmed the practical effectiveness of Japanese swords in the close-quarters fighting that defined samurai combat in this era. The great masters of late Kamakura sword-making — Nagamitsu of Osafune, Kuniyuki and Kuniyoshi of Yamashiro, and Masamune of Sagami — were producing blades at the peak of their craft. The national treasure Daihanya Nagamitsu, made around this period, exemplifies the robust, wide-bladed tachi that Kamakura warriors actually carried into battle. The victory of 1281, attributed partly to the "divine wind" (kamikaze), reinforced the spiritual significance of the sword in Japanese culture: many warriors dedicated swords to shrines in thanksgiving — a practice that has preserved some of Japan's finest medieval blades to the present day.