In the early summer of 1560, Imagawa Yoshimoto (今川義元) assembled one of the largest armies seen in central Japan in a generation. Roughly 25,000 men — soldiers, cavalry, arquebusiers, and a vast supply train — moved west along the Tōkaidō highway, the road that connected the eastern provinces to the imperial capital at Kyoto. The Imagawa controlled three provinces: Suruga (駿河), Tōtōmi (遠江), and Mikawa (三河). They had the men, the money, and the momentum. Yoshimoto, 41 years old and at the peak of his power, was the man most likely to reshape Japan.

In his path, almost incidentally, stood Oda Nobunaga (織田信長). The young lord of Owari Province (尾張国) was known more for his unpredictability than his battlefield record. He had spent the late 1550s fighting rival branches of his own clan, clawing together control of a single small province. His army numbered perhaps 2,000 to 3,000 effective troops. From Yoshimoto's perspective, Nobunaga was not an adversary — he was an obstacle, to be brushed aside on the march to glory.

The Imagawa vanguard, under Matsudaira Motoyasu (松平元康) — a young man who would one day be known as Tokugawa Ieyasu (徳川家康) — reached Odaka Castle on June 10. The castle's castellan had already defected to the Imagawa cause. Yoshimoto's forces were not merely approaching Owari; they were already inside it. On June 11, Yoshimoto held a war council at Kutsukake and ordered his vanguard to strike the Oda frontier forts at Narumi (鳴海), Washizu (鷲津), and Marune (丸根) simultaneously, to clear the road west.

Inside Kiyosu Castle (清洲城), the reports arriving at Nobunaga's door were increasingly desperate. Messengers described columns of soldiers stretching for miles along the highway. His senior retainers — men who had survived decades of Sengoku violence and knew what armies of that size could do — gathered around him and begged him to stand and hold. The calculation was not cowardice; it was arithmetic. The castle was defensible. The odds in open battle were catastrophic. Every rule of Sengoku warfare said the same thing: wait.

Nobunaga broke up the council and sent his men home. According to later accounts, he told them that to wait behind walls was merely a slower form of death — that the only chance was to attack, and attack immediately. His retainers dispersed into the summer night, almost certainly convinced their lord had lost his mind.

What Nobunaga understood that the others did not — or would not let themselves believe — was terrain and weather. The Imagawa army would have to pass through a narrow, gorge-like valley near Okehazama village (桶狭間). In such a place, 25,000 men could not maneuver as 25,000 men; they would be strung out and confined, their numbers no longer an asset. And June in Owari Province meant violent afternoon thunderstorms — storms that could blind a marching column, drown out the sound of approaching feet, and transform overconfidence into vulnerability in minutes. Nobunaga was not gambling against the odds. He was gambling that the odds could be changed.

The night before he led his men out, the chronicles record that Nobunaga danced and sang the kowakamai (幸若舞) ballad 敦盛 (Atsumori), a lament about a young warrior killed before his time. Its most famous couplet: "Man has but 50 years; compared to the lower realm, his time is a dream and an illusion. Is there any who, once born, does not die?" Whether he slept after that performance, no source says. What is clear is that when morning came, Oda Nobunaga rode out with a force barely a tenth the size of the army that was coming to destroy him — and he was smiling.