In alliance with the Republic of Ragusa, Michael II Asen broke into Serbia in 1254, but he could not occupy Serbian territories.

[1] According to a scholarly theory, Michael's mother ruled Bulgaria during his minority, but she stayed in Thessaloniki just three months after his coronation.

[3][5] Michael II Komnenos Doukas, ruler of Epirus, also broke into Bulgaria and occupied western Macedonia.

[9] Despite the tense relationship between Hungary and Bulgaria, Bulgarian troops assisted Vatatzes to invade the Thracian territories of the Latin Empire in August 1247.

[15] Taking advantage of the absence of significant Nicene forces, Michael broke into Macedonia and reconquered the lands lost to Vatatzes in 1246 or 1247.

[15][16] The Byzantine historian, George Akropolites, recorded that the Bulgarian-speaking local inhabitants supported Michael's invasion because they wanted to shake off the "yoke of those who spoke another language".

[20] When referring to the new war between Nicea and Bulgaria, Rubruck described Michael as "a mere lad whose power has been eroded"[21] by the Mongols.

[22] A rebellion at Melnik forced Theodore to march towards the town, but he managed to defeat the rioters before returning to Asia Minor for the winter.

[23] Theodore agreed to sign a peace treaty only after Michael acknowledged the loss of the lands that he had claimed for Bulgaria.