From wikipedia :

In this battle, the heavily outnumbered Oda clan troops, commanded by Oda Nobunaga, defeated Imagawa Yoshimoto and established Oda as one of the front-running warlords in the Sengoku period.

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The actual sequence of events of the Battle of Okehazama has been much embellished by legend and is historically unclear. Traditionally it has been held that the vast discrepancy in numbers caused Nobunaga and his men on June 12 to secrete themselves in an area known as Kamagatani on the other side of the Imagawa main camp, and the Oda forces made a flanking maneuver and attacked the Imagawa army at Dengaku-hazama from the north. However, familiarity of the Oda forces with the terrain and Nobunaga’s propensity for aggressive tactics make many modern historians theorize that the attack was actually a frontal assault on Yoshimoto’s camp, either by design or by accident.

In any event, the Imagawa army was caught by surprise. The Imagawa were celebrating their recent easy victories, and as the afternoon was very hot, many had removed their armor. Using a thunderstorm to mask their movements, the Oda troops struck hard at the heart of the Imagawa camp, which was in a narrow valley. The surprise attack caused a panic and the Imagawa troops broke ranks and many attempted to flee.