Mahmud's brother Mihailo became member of a collective regency, but he was soon deposed by the anti-Ottoman and pro-Hungarian faction in the Serbian court. In reaction, Mahmud attacked and seized Smederevo Fortress, although the citadel held out, and seized some additional strongholds in its vicinity. Threatened by a possible Hungarian intervention however he was forced to withdraw south and join the forces of Sultan Mehmed II at Skopje.[14] In 1461, he accompanied Mehmed in his campaign against the Empire of Trebizond, the last surviving fragment of the Byzantine Empire.