Rostislav Mikhailovich ya kai wa Bulgaria hari tare da taimakon ɗan ƙasar Hungary a shekara ta 1259. A shekara ta gaba, Rostislav ya bar ƙungiyarsa don ya shiga yaƙin da surukinsa, Béla IV na Hungary, ya yi da Bohemia.Yin amfani da rashi na Rostislav, Konstantin ya shiga cikin mulkinsa kuma ya sake mamaye Vidin.Ya kuma aika da sojoji su kai farmaki a Banate na Severin, amma kwamandan Hungarian, Lawrence, ya yi yaƙi da maharan.Mamaya na Bulgaria na Severin ya fusata Béla IV.Ba da daɗewa ba bayan da ya kulla yarjejeniyar zaman lafiya da Ottokar II na Bohemia a cikin Maris 1261, sojojin Hungary sun kutsa cikin Bulgaria a ƙarƙashin umarnin ɗan Béla IV kuma magaji, Stephen.Sun kama Vidin kuma suka kewaye Lom a kan ƙananan Danube, amma ba su iya kawo Konstantin zuwa yaƙi ba, saboda ya koma Tarnovo.Sojojin Hungary sun bar Bulgaria kafin karshen shekara, amma yakin ya mayar da arewa maso yammacin Bulgaria zuwa Rostislav.