The Byzantine Emperor Michael VIII Palaiologos decided to make use of the instability in Bulgaria. He sent an army to impose his ally Ivan Asen III on the throne. Ivan Asen III gained control of the area between Vidin and Cherven. Ivailo was besieged by the Mongols at Drastar (Silistra) and the nobility in the capital Tarnovo accepted Ivan Asen III for Emperor.

In the same year, however, Ivailo managed to make a breakthrough in Drastar and headed for the capital. In order to help his ally, Michael VIII sent a 10,000-strong army towards Bulgaria under Murin. When Ivailo learned of that campaign he abandoned his march to Tarnovo. Although his troops were outnumbered, the Bulgarian leader attacked Murin in the Kotel Pass on 17 July 1279 and the Byzantines were completely routed. Many of them perished in the battle, while the rest were captured and later killed by orders from Ivailo. After the defeat Michael VIII sent another army of 5,000 troops under Aprin but it was also defeated by Ivailo before reaching the Balkan Mountains. Without support, Ivan Asen III had to flee to Constantinople.