During the civil war in Hungary, Stephen's vassal, Despot Jacob Svetoslav submitted himself to Tsar Constantine Tikh of Bulgaria. In the summer of 1266, Stephen invaded Bulgaria, seized Vidin, Pleven and other forts and routed the Bulgarians in five battles. Jacob Svetoslav again accepted Stephen's suzerainty and was reinstalled in Vidin. From then on, Stephen used the title "King of Bulgaria" in his charters.
During the civil war in Hungary, Stephen's vassal, Despot Jacob Svetoslav submitted himself to Tsar Constantine Tikh of Bulgaria. In the summer of 1266, Stephen invaded Bulgaria, seized Vidin Pleven and other forts and routed the Bulgarians in five battles. Jacob Svetoslav again accepted Stephen's suzerainty and was reinstalled in Vidin. From then on, Stephen used the title "King of Bulgaria" in his charters.