When 今川義元 (Imagawa Yoshimoto) departed 駿河 (Suruga) Province in the summer of 1560, his campaign had the character of a procession rather than a gamble. He commanded approximately 25,000 men — some sources put the figure as high as 35,000 — against a 尾張 (Owari) defense that could muster somewhere between 2,000 and 2,500 fighters. The road to Kyoto ran through 尾張 (Owari), and 今川 (Imagawa) intended to walk it. 足利将軍 (Ashikaga shogun)'s weakened court awaited at the other end; with enough force behind him, Yoshimoto could dictate terms to the capital and reshape the balance of 戦国時代 (Sengoku Jidai) power in his favor.

The 織田 (Oda) border forts fell in sequence as the campaign advanced. 今川 (Imagawa) vanguard forces overran one fortified position after another; none could resist for long at such disparity of numbers. By the time the main army settled into camp at a narrow forested valley called 桶狭間 (Okehazama) — sometimes written 田楽狭間 (Dengakuhazama) in older sources — the atmosphere in the 今川 (Imagawa) camp was not tense anticipation but something approaching celebration. 義元 (Yoshimoto)'s generals had reportedly been assigned specific 尾張 (Owari) territories as their rewards once the campaign concluded — naming districts and domains as though the allocation were an administrative exercise rather than a military aspiration. This was 油断 (yudan): the guard lowered, the lamp going out before the journey was done.

織田信長 (Oda Nobunaga) was 25 years old. He had not yet turned 26 — his birthday fell eleven days after the battle. He was not widely feared, not widely respected, and by any rational accounting had no path to victory. By one account, before departing with his small force, he performed the 能楽 (Nogaku) noh dance 「敦盛」 (Atsumori) and spoke on the brevity of human life: a man's lifetime is fifty years; compared to the cycle of heaven and earth it is a dream. Then he marched.

The thunderstorm was the wild card that no calculation in the 今川 (Imagawa) camp had accommodated. It struck around midday — sudden, violent, with thunder and lightning and heavy rain — reducing visibility across the 桶狭間 (Okehazama) valley to near zero. Sound became impossible. The enormous 今川 (Imagawa) army, strung out for miles along the march route, had no way to relay information rapidly across its length when the center was struck. Its very size had become a liability.

信長 (Nobunaga) led his force of roughly 2,000 troops in a rapid flanking attack through the storm, crossing several kilometers of terrain without being detected. When they burst into the 今川 (Imagawa) camp, the confusion was absolute. 義元 (Yoshimoto) reportedly thought the commotion was a drunken brawl among his own men. By the time he understood what was happening, 織田 (Oda) samurai were already upon him.

To his credit, 義元 (Yoshimoto) was not helpless. He seized a weapon and fought back, wounding the first Oda samurai — 森可成 (Mori Yoshinari), some sources say 毛利新介 (Mori Shinsuke) — who reached him, cutting through the man's spear thrust. He ran out of luck on the second attacker, who tackled him to the ground. 服部小平太 (Hattori Koheita) took his head. The man who had been marching to Kyoto as a near-certainty died in a narrow valley in a thunderstorm, his vast army still arrayed around him, unable to respond before it was over.

The 25,000-man army dissolved. Without its commander it had no purpose, no direction, and no will to continue. The border lords who had been aligned with 今川 (Imagawa) quickly re-evaluated their loyalties. Among them was 松平元康 (Matsudaira Motoyasu), then an 今川 (Imagawa) retainer — the young man who would later become 徳川家康 (Tokugawa Ieyasu), and whose eventual partnership with 信長 (Nobunaga) would shape the next fifty years of Japanese history. At 桶狭間 (Okehazama), all of that was still in the future. What was immediate was this: a 25-year-old nobody had just killed the most powerful 大名 (daimyo) in eastern Japan, in a thunderstorm, with 2,000 men. 信長 (Nobunaga)'s name spread across the country overnight.