In the turbulent Sengoku era of 16th-century Japan, rival daimyō built vast coastal fleets to control inland seas. The most formidable vessels were the atakebune—floating fortresses propelled by oars, with towering decks bristling with arquebusiers and a handful of small cannons. Oda Nobunaga led the way in 1578, commissioning six “iron-covered ships” (tekkōsen) whose superstructures he reinforced with iron plates to resist enemy cannon and fire arrows. These hulking crafts proved decisive that year when Nobunaga’s navy bottled up the Mōri at the Kizu River’s mouth, cutting off supplies and securing Osaka’s coastline. Under Toyotomi Hideyoshi, Japan’s maritime ambitions grew bolder. In preparation for the 1592 invasion of Korea, Hideyoshi ordered true ironclads—ships with decks, bulwarks and ramps sheathed entirely in iron. One measured 36.3 m long and gleamed with gilded fittings. Yet their heavy ribs proved brittle: some split and sank under stress. In Korean waters, the atakebune’s weaknesses surfaced. Korea’s sturdy panokseon warships carried many more cannons and traded blows at range, thwarting the Japanese tactic of boarding. Even Hideyoshi’s ironclads bowed to superior Korean firepower and hull strength. After unification under Tokugawa Ieyasu, Japan built grand atakebune like the copper-clad Atakemaru for Hidetada and Iemitsu. But with peace established, these floating fortresses faded from use. By the early 17th century, Japan’s naval focus shifted away from coastal ironclads toward diplomacy and trade.

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Atakebune