|Previous: Battle of Mt. Kogi|
|Next: Suruga-Sagami Conflict|
|Siege of Kawagoe Castle|
|Conflict: Sengoku Jidai|
|Date: October 1545-19 May 1546|
|Place: Musashi Province|
|Outcome: Hojo victory|
16,000 killed or wounded
Ujiyasu Hojo was the third-generation lord of the Hojo clan of Sagami Province, which had been founded by Hojo's grandfather. Ujiyasu's father Soun Hojo had established the Hojo as one of the most powerful factions in eastern Japan, but, unlike his father and grandfather, Ujiyasu was known as the "Fool of Sagami". Seeing an opportunity to eliminate the Hojo clan, several rival warlords banded together and attacked Kawagoe Castle. The 80,000-strong coalition laid siege to the 3,000-strong garrison, which was later reinforced by 8,000 troops. Ujiyasu sent numerous messengers to the Uesugi requesting a truce, convincing the Uesugi forces that he sought to surrender. However, he instead had his men remove their helmets and armor and embark on a surprise attack against the coalition forces.
The coalition generals Norishige Obata, Norikage Nagao, and Norinaga Nagao ventured too far north, so Ujiyasu sought to eliminate them before ambushing Norimasa Uesugi. Afterwards, the Hojo relief force would reinforce Kawagoe Castle, and the Hojo army would proceed to defeat Tomosada Uesugi and conquer the Kanto region.
Ujiyasu's surprise attack annihilated the coalition vanguard, and he then cut down Norimasa after capturing the southeast garrison. He then had his men unleash a battle cry, panicking the Coalition besiegers. Infighting broke out among the besiegers, with Noriyori Takada, Harusuke Yanada, and Norihiro Uesugi rebelling. Ujiyasu made his way to the castle and decided to evacuate Kawagoe Castle, escorting Tsunashige Hojo to the Igusa stronghold. At that time, Shingen Takeda and Yoshimoto Imagawa arrived with their own armies to assist the besiegers, and Ujiyasu was able to quickly repel his brother-in-law Yoshimoto's forces. Ujiyasu then defeated the Takeda army as Tsunashige headed north to secure Koga, which would deliver Kanto into the hands of the Hojo clan. Now, Ujiyasu headed to strike the killing blow, assaulting Tomosada's camp. Tomosada was killed in battle, ending the Ogigayatsu clan and ending the siege. Ujiyasu was now in control of the Kanto region, and the Hojo clan remained a major powerbroker in eastern Japan.