The Battle of Mimase Pass took place on Oct. 8, 1569 between the armies of the Houjou Clan, led by Houjou Ujiteru and Houjou Ujikuni, and the Takeda Clan, led by Takeda Shingen, after the Kai-Sagami-Suruga alliance fell apart in 1568 upon Takeda's invasion of Suruga.

On Oct. 1, 1569, Takeda Shingen besieged Houjou Ujiyasu's Odawara Castle with 20,000 troops, but like Uesugi Kenshin before him, failed to penetrate the Houjou Clan's main fortress' defenses and retreated four days later on Oct. 5 after setting fire to the land near the castle. Houjou Ujiteru and Houjou Ujikuni set up an ambush with an army of 12,000-20,000 (estimates differ) at the strategic Mimase Pass for the Takeda army as it retreated back towards Kai. Houjou Ujimasa brought up their main army of 20,000 in a pincer movement.

Takeda Shingen led the Takeda army along with his generals Takeda Katsuyori, Takeda Nobukado, Yamagata Masakage, Naitou Masatoyo, Baba Nobuharu, Asari Nobutane, and Obata Norishige.

Houjou Ujiteru and Houjou Ujikuni led the Houjou army along with their generals Houjou Ujitada, Takajou Kurando (?), Hara Tanehide, and Ueda Tomonao.

Ujiteru and Ujikuni apparently sprung the ambush before Ujimasa arrived. Shingen, who had sensed the attack, split his army into three parts: one met the Houjou head-on, while the other two hid in the mountain and attacked from the side. The two armies met in full-blown battle on Oct. 8. The Houjou held the advantage at the start. However, a detached force led by Yamagata Masakage struck in a surprise attack from the higher ground of Shida Pass about a kilometer to the south-west, managing to turn the tide of the battle. The Takeda army also employed arquebuses in the narrow mountain pass to their great advantage, and the Houjou army suffered heavy losses. Though the fighting was close at the beginning of the battle, by the end it was clearly a Takeda victory.

Ujimasa halted his army upon hearing of the Houjou defeat, so the pincer attack (which may yet have won the day for the Houjou) never came to pass. Takeda then pushed forward into Sagami, there to raise his victory shout, before retreating.

Because Uesugi Kenshin had not sent aid during the seige and battle, the Houjou Clan began to view him with distrust, and Houjou Ujiyasu wrote a letter to him expressing his discontent. This battle would become the underlying cause for the Houjou's severing of their alliance with the Uesugi Clan and reformation of their alliance with the Takeda Clan.