Rostislav Mikhailovich invaded Bulgaria with Hungarian assistance in 1259. In the following year, Rostislav left his duchy to join the campaign of his father-in-law, Béla IV of Hungary, against Bohemia. Taking advantage of Rostislav's absence, Konstantin broke into his realm and reoccupied Vidin. He also sent an army to attack the Banate of Severin, but the Hungarian commander, Lawrence, fought the invaders off.

The Bulgarian invasion of Severin outraged Béla IV. Soon after he concluded a peace treaty with Ottokar II of Bohemia in March 1261, Hungarian troops stormed into Bulgaria under the command of Béla IV's son and heir, Stephen. They captured Vidin and besieged Lom on the Lower Danube, but they were unable to bring Konstantin to a pitched battle, because he withdrew to Tarnovo. The Hungarian army left Bulgaria before the end of the year, but the campaign restored northwestern Bulgaria to Rostislav.