In 1560, Imagawa Yoshimoto gathered an army of 25,000 men[1][3] and started his march toward Kyoto, with the excuse of aiding the frail Ashikaga shogunate. The Matsudaira clan of Mikawa Province was also to join Yoshimoto's forces. In comparison, the Oda clan could rally an army of only 1,800[citation needed], and the forces would also have to be split up to defend various forts at the border. Under such dire circumstances, Nobunaga was said to have performed his favorite Atsumori dance, before riding off with only a few attendants to pray in a shrine.

The Oda clan's generals did not believe that they would survive the attack from Imagawa Yoshimoto's army. Only the night before, Shibata Katsuie had tried in vain to change Oda Nobunaga's mind about a frontal attack; he kept reminding Nobunaga of the joint army's complete lack of manpower compared to the Imagawa soldiers, who, according to rumors, numbered 40,000 men[citation needed]. Hayashi Sado no Kami Hidesada, the remaining advisor from Nobuhide's days, even argued for surrender without fighting, using the same reasoning as Katsuie[citation needed].

Nobunaga's scouts reported that Yoshimoto was resting his troops at a place called Dengaku-hazama, near a small village called Okehazama. Nobunaga knew the countryside well. Dengaku-hazama was a narrow gorge, an ideal place for a surprise attack if the conditions were right. The scouts added that the Imagawa army were celebrating their victories with food and drink while Yoshimoto viewed the heads. Nobunaga moved up towards Imagawa's camp, and set up a position some distance away.[where?] An array of flags and dummy troops made of straw and spare helmets gave the impression of a large host, while the real Oda army hurried round in a rapid march to get behind Yoshimoto's camp. Fortune and weather favored Nobunaga, for about mid-day the stifling heat gave way to a terrific thunderstorm. As the Imagawa samurai sheltered from the rain Nobunaga deployed his troops, and when the storm ceased they charged down upon the enemy in the gorge. So sudden was the attack that Yoshimoto thought a brawl had broken out among his men. He realized it was an attack when two samurai (Mōri Shinsuke and Hattori Koheita)[4] charged up. One aimed a spear at him, which Yoshimoto deflected with his sword, but the second swung his blade and cut off Imagawa's head.[citation needed]

Rapidly weakening, the Imagawa clan no longer exerted control over the Matsudaira clan. In 1561, an alliance was forged between Oda Nobunaga and Matsudaira Motoyasu (later Tokugawa Ieyasu), despite the decades-old hostility between the two clans. Tradition dates this battle as the time that Nobunaga first noticed the talents of the sandal bearer who would eventually become Toyotomi Hideyoshi.