The morning of June 12, 1560 began as a Tokaidō (東海道) march of conquest. Imagawa Yoshimoto (今川義元) had spent the prior days methodically destroying the Oda frontier defenses. His vanguard had stormed the fort at Marune (丸根), killing the Oda commander Sakuma Morishige (佐久間盛重), and taken Washizu (鷲津) as well. The road to Kiyosu — and beyond it, to Kyoto — was opening. Yoshimoto paused at the gorge of Dengakuhazama (田楽狭間) in Owari Province (尾張国) to rest his men and review the battle dispatches. He was 41 years old, at the apex of his career, and his army of approximately 25,000 men surrounded him in every direction. Security was not a concern. Victory was already in motion.

Yoshimoto was not merely a military figure. He was a man of considerable cultivation — practiced in kemari (蹴鞠), the ancient court football; learned in poetry and calligraphy; known for the aristocratic habit of blackening his teeth and powdering his face in the Kyoto fashion. He represented, in his person, the ideal of the warrior-aristocrat: a great lord who was also a great gentleman. Sitting in his camp that afternoon, waiting out the heat, he was precisely the kind of figure who could not imagine being killed in the mud by strangers he had never heard of.

Inside Kiyosu Castle (清洲城), Oda Nobunaga (織田信長), 25 years old, had spent the night dismissing his senior retainers' pleas for caution and sending them home to rest before dawn. He had danced and sung the 敦盛 (Atsumori) ballad. And when morning came, he led somewhere between 2,000 and 3,000 men — roughly a tenth of Yoshimoto's force — east toward the gorge that every experienced commander in his castle would have told him to avoid entirely.

In the early afternoon, the skies over Owari broke open. A violent summer thunderstorm — the kind that Nobunaga had understood would come, because June in Owari was June in Owari — struck the hills around Dengakuhazama. Lightning cracked. Rain fell in sheets. The noise alone was enough to drown out the sound of several thousand men moving quickly through the undergrowth. Yoshimoto's troops, resting after a morning of fighting and marching, could not hear what was coming.

When the Oda force burst into the Imagawa camp, Yoshimoto reportedly looked up at the commotion and assumed it was a drunken fight among his own men. It was a reasonable assumption — his force was enormous, and minor brawls were a constant of military life. By the time he understood what was happening, there was already a spear in his face.

Mori Shinsuke (毛利新介) struck first. Yoshimoto parried. Despite being ambushed in a camp that had dissolved into chaos, the lord of three provinces responded like the warrior he was — he cut through the spear shaft and wounded Mori in the knee. One more defender, one second more of reaction time, and this story ends differently. But Hattori Koheita (服部小平太) was already moving. He tackled Yoshimoto and took his head.

The death of its commander destroyed the Imagawa army as a coherent force. Senior officers — many of them killed in the initial assault — were gone. The men who survived fled east, back toward Imagawa territory, the great march to Kyoto abandoned. Within years, Yoshimoto's three provinces were divided among his neighbors, and the Imagawa, once the dominant power on the Tōkaidō road, ceased to matter.

One young man who had been in the Imagawa vanguard that day — Matsudaira Motoyasu (松平元康), the future Tokugawa Ieyasu (徳川家康) — walked away from the battle and eventually toward an alliance with Nobunaga. He had spent his childhood as an Imagawa political hostage; Yoshimoto's death set him free. The thunderstorm at Okehazama (桶狭間) did not just kill one warlord. It released another.