When the Treaty of Amity and Commerce (United States–Japan) was signed in 1858 (Ansei 5), Yoshikatsu joined with his uncle and cousin—Tokugawa Nariaki and his son Yoshiatsu—together with Matsudaira Yoshinaga (daimyō of the Fukui Domain) and others in making an unsolicited visit to Edo Castle, where they oppose the policies of the Tairō, Ii Naosuke. This act was censured, everyone who opposed Ii Naosuke was purged (the Ansei Purge). For his part Yoshikatsu was placed under house arrest, and he was replaced by his younger brother Mochinaga, who thus became the 15th daimyō of Owari.