Michael Asen I of Bulgaria attempted to conquer land taken by the Empire of Nicaea, but the advance of Theodore II Lascaris caught the Bulgarians unprepared.

[1] In 1252, the Byzantine Emperor John III Doukas Vatatzes led a successful campaign against Epirus, seizing the towns of Kastoria, Ohrid, Prilep, and Vodena.

By this point the Byzantines had a stranglehold on Constantinople, and Vatatzes made plans for its capture.

However, Vatatzes died in 1254, and was succeeded by his son Theodore II Laskaris, who had epilepsy and was often ill.[2] Shortly after his ascension to the throne, Theodore II found the Empires' newly won possessions in Thrace under attack by Michal Asen I of Bulgaria.

[2] After he assembled his troops, he himself led them into battle, successfully repelling the Bulgarians and preserving the Empires' borders.