To overcome his failure to secure gains against the Serbians, Andronikos III attempted to annex Bulgarian Thrace, but the new tsar Ivan Alexander of Bulgaria defeated Byzantine forces at Battle of Rusokastro on 18 July 1332. Territorial concessions and a diplomatic marriage between the son of the Bulgarian emperor, the future Michael Asen IV of Bulgaria, and Maria (renamed Irene) Palaiologina, daughter of Andronikos III Palaiologos, secured peace with Bulgaria.