On the morning of May 6, 1869, three warships belonging to the 蝦夷共和国 (Ezo Kyōwakoku, Republic of Ezo) rounded the headland into Miyako Bay on the Pacific coast of Honshu. The lead ship, 回天 (Kaiten), flew the American flag. To the Imperial Navy crews aboard the vessels anchored in the bay — including the prized ironclad 甲鉄 (Kotetsu) — there was nothing alarming about an American ship entering a Japanese harbor. Commerce and diplomacy with Western nations were routine. They watched without alarm.

The deception was the plan's only moment of genius. The 蝦夷共和国 (Ezo Kyōwakoku, Republic of Ezo) had been losing since January. This breakaway government — formed by shogunate loyalists who refused to accept the Meiji Restoration — controlled Hokkaido and the fortress of 五稜郭 (Goryōkaku) at Hakodate, but little else. Its navy was outgunned. The Kotetsu, an American-built ironclad steam warship recently acquired by the Imperial government, gave the Meiji forces naval supremacy the Ezo Republic could not match. Without their own ironclad, the Republic's remaining warships were obsolete. The plan was audacious: enter under a false flag, ram the Kotetsu, and take her by boarding.

French advisors helped devise the operation. 欧仁・コラッシュ (Eugène Collache), aboard the second ship 高雄 (Takao), had helped plan the assault. アンリ・ニコル (Henri Nicol), a former French Navy officer embedded with the Ezo forces, was aboard Kaiten itself. Their presence underlined the desperation of the Republic's position — grasping at any foreign expertise that might tip the balance.

When Kaiten closed to ramming distance, the American flag came down. The Ezo Republic's banner went up. The prow struck Kotetsu's iron hull. The boarding party — samurai armed with swords and firearms — surged to the rail. And there the plan died.

Nobody had adequately accounted for the difference in deck height. Kaiten's deck rose nearly three meters above Kotetsu's. Instead of a mass boarding surge that could overwhelm the ironclad's crew before a defense could be organized, each man had to drop down individually onto the lower deck. One at a time. Into a defended space. The Kotetsu's crew brought a Gatling gun to bear — an American-made weapon capable of firing 200 rounds per minute, something few of the boarders had ever faced in combat. The deck became a killing ground.

甲賀源吾 (Kōga Gengo), born February 16, 1839, was 30 years old on the morning he died. He was captain of Kaiten and led the boarding party personally. Shot through the left foot, he kept moving. Shot through the right arm, he kept moving. Then shot through the head. He was dead on the deck of a ship he almost captured.

Also aboard Kaiten that morning was 土方歳三 (Hijikata Toshizō), 33, the legendary vice-commander of the 新選組 (Shinsengumi) — the shogunate's elite police force, feared throughout the Bakumatsu era as enforcers of order in Kyoto. Whether Hijikata personally joined the boarding attempt is unclear from the sources, but he was present on Kaiten during the battle. He had survived every catastrophe of the Boshin War to reach this moment. He would survive Miyako Bay too, only to be killed six weeks later on June 20, 1869, at the final battle of 箱館 (Hakodate).

The three Ezo warships eventually withdrew. Kaiten had managed to damage three Imperial vessels with gunfire before disengaging, but the Kotetsu was unharmed, still in Imperial hands. Admiral 荒井郁之助 (Arai Ikunosuke), who took command after Kōga was killed, brought what remained of the boarding force back to the ships.

The Republic of Ezo surrendered 51 days later, on June 27, 1869, when Goryōkaku fortress fell. This was the first attempted boarding of an ironclad steamship in Japanese naval history — and its failure made the outcome of the Boshin War inevitable. The Kotetsu's Gatling gun had said something that could not be unsaid: samurai courage was no longer enough.