Vatatzes died on 4 November 1254. Taking advantage of the absence of significant Nicene forces, Michael broke into Macedonia and reconquered the lands lost to Vatatzes in 1246 or 1247. 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".

Theodore II Laskaris, launched a counter-invasion in early 1255. When referring to the new war between Nicea and Bulgaria, Rubruck described Michael as "a mere lad whose power has been eroded" by the Mongols. Michael could not resist the invasion and the Nicene troops captured Stara Zagora. It was only the harsh weather that prevented Theodore's army from continuing the invasion. The Nicene troops resumed their attack in the spring and occupied most fortresses in the Rhodope Mountains. 

Michael broke into the European territory of the Empire of Nicea in the spring of 1256. He pillaged Thrace near Constantinople, but the Nicene army defeated his Cuman troops. He asked his father-in-law to mediate a reconciliation between Bulgaria and Nicea in June. Theodore agreed to sign a peace treaty only after Michael acknowledged the loss of the lands that he had claimed for Bulgaria. The treaty determined the upper course of the river Maritsa as the border between the two countries. The peace treaty outraged many boyars (noblemen) who decided to replace Michael with his cousin, Kaliman Asen. Kaliman and his allies attacked the Tsar who died from his wounds in late 1256 or early 1257.