Ivan Asen II ya gaje shi da ɗansa Kaliman I. Duk da nasarar farko a kan Mongols , mulkin sabon sarki ya yanke shawarar kauce wa ƙarin hare-hare kuma ya zaɓi ya biya su haraji maimakon.Rashin samun wani sarki mai karfi da karuwar hamayya a tsakanin masu fada aji ya sa Bulgaria ta yi saurin raguwa.Babban abokiyar hamayyarta Nicaea ta guje wa hare-haren Mongol kuma ta sami iko a cikin Balkans.Bayan mutuwar Kaliman I ɗan shekara 12 a shekara ta 1246, wasu ƴan gajeruwar sarauta da dama ne suka gaje shi.An fallasa raunin sabuwar gwamnati sa’ad da sojojin Nicaea suka ci manyan yankuna a kudancin Thrace, Rhodopes, da Makidoniya—har da Adrianople, Tsepina, Stanimaka, Melnik, Serres, Skopje, da Ohrid—sun yi tsayayya da juna.Har ila yau, ' yan Hungary sun yi amfani da raunin Bulgarian, sun mamaye Belgrade da Braničevo.