Establishment of the Kosun Alliance lead to the break of the Sunso Alliance (Suruga-Sagami alliance), an alliance that had been with a long time relative family, the Gohojo clan, who were dominating the provinces of Izu (Izu region, Shizuoka Prefecture) and Sagami (southwest part of Kanagawa Prefecture), resulting in Ujitsuna HOJO setting out for a military invasion into Yoshihara of Kato County in Suruga Province (Kato War).
甲駿同盟の成立は旧来の縁戚であった伊豆国（静岡県伊豆地方）・相模国（神奈川県西南部）を領する後北条氏との駿相同盟決裂を招き、北条氏綱が駿河河東郡吉原に侵攻する（河東の乱）。