What was the Battle of Okehazama?
At Okehazama, Oda Nobunaga attacked the headquarters of a much larger invading army and killed its commander, Imagawa Yoshimoto, in a single morning. It is the most famous upset victory in Japanese history.
The battle made Nobunaga a national figure, destroyed the strongest power on the road between Kyoto and eastern Japan, and released the young Tokugawa Ieyasu — then an Imagawa subordinate — to become an independent lord.
Why did it happen?
Yoshimoto controlled Suruga, Totomi and Mikawa, and in 1560 marched west into Owari. The traditional explanation is that he was heading for Kyoto to assert national authority; many historians now think the immediate aim was more limited, namely securing the contested Owari border.
Nobunaga had only recently unified Owari and could field a fraction of the Imagawa numbers. His senior retainers reportedly favoured defending the main castle at Kiyosu. He chose to attack instead.
Forces and sides
The figures usually given are 25,000 or more for the Imagawa against 2,000 to 3,000 for the Oda. The Imagawa number comes from later chronicles and is very likely inflated; the total Imagawa army may have been closer to 20,000, with only part of it near the command post.
The important asymmetry is not the total but the local one: Nobunaga struck a fraction of the Imagawa army with most of what he had.
What happened?
The Imagawa advance took two Oda border forts on the morning of the battle. Yoshimoto halted his headquarters in a narrow area of low hills at Okehazama to rest, and — by the standard account — to hold a ceremony for the heads taken that morning.
Nobunaga left Kiyosu at dawn with a small force, stopped at Atsuta Shrine, and moved on the Imagawa position. A violent thunderstorm broke over the area in the early afternoon.
What happened next is the disputed part. Edo-period accounts describe a concealed flanking march through the hills and a surprise attack from the rear. The earliest and most reliable source, the Shincho Koki written by a man in Oda service, describes something more direct: Nobunaga moved onto the high ground and attacked the camp frontally as the storm cleared, catching an army that was resting and had no idea he was close.
Yoshimoto's guard was overrun within a short time. He was killed by Oda troops; the man usually credited with taking his head is Mori Shinsuke.
Turning point
The decisive factor was that Yoshimoto's command post was separated from the bulk of his army and unprepared. Once he was dead, the Imagawa force had no reason and no mechanism to keep fighting.
Outcome
The Imagawa army broke up and went home. Yoshimoto's son Ujizane could not hold the alliance together, and within a decade the Imagawa provinces had been divided between the Takeda and the Tokugawa.
Ieyasu, who had been fighting as an Imagawa subordinate, returned to Mikawa, took his family castle at Okazaki, and by 1562 had allied with Nobunaga.
Why it mattered
Okehazama is where the unification of Japan becomes conceivable. It removed the strongest bloc in central Japan in a single morning and created the Oda–Tokugawa alliance that held for the next twenty years.
It also fixed Nobunaga's reputation for doing the aggressive, unreasonable thing and being right. That reputation was worth armies to him later.
