In the mid-1550s, the Imagawa, Hojo, and Takeda clans met at the Zentoku-ji temple in Suruga province and established a peace treaty. The proceedings were moderated by a monk named Taigen Sessai. The three daimyo agreed not to attack each other, as well as made agreements on support and reinforcements if necessary. This agreement was held together by three marriages - Hojo Ujimasa married the daughter of Takeda Shingen (Obai-in), Imagawa Ujizane married a daughter of Hojo Ujiyasu, and Takeda Yoshinobu had already married the daughter of Imagawa Yoshimoto in 1552, further strengthening ties between the Takeda and the Imagawa. Due to these agreements, the three daimyo were able to focus on their own goals without fear of attack.
This peace treaty laid the foundation for Imagawa Yoshimoto to focus on the Oda clan in Owari, and attempt a move on Kyoto. The Hojo clan benefitted by being freed up to expand their own influence in the Kanto, and Takeda Shingen was able to focus on his invasion of Shinano province.
The Japanese name of the alliance, 甲相駿三国同盟, literally means "Alliance of the three provinces of Kai (Takeda), Sagami (Hôjô), and Suruga (Imagawa)."
If Imagawa Yoshimoto had not made peace with the Hojo and Takeda, he may not have invaded Owari in 1560 to be killed by Oda Nobunaga. Had that not happened, the entire history of the Sengoku may have changed completely.