Ivailo of Bulgaria attacked the Byzantine army sent to help his rival for the crown Ivan Asen III.
[3] In 1277 in a popular uprising led by Ivailo broke out in north-eastern Bulgaria against the incapability of Emperor Constantine Tikh Asen to cope with the constant Mongol invasions which devastated the country for years.
Ivailo was besieged by the Mongols at Drastar (Silistra) and the nobility in the capital Tarnovo accepted Ivan Asen III for Emperor.
Although his troops were outnumbered, the Bulgarian leader attacked Murin in the Kotel Pass on 17 July 1279 and the Byzantines were completely routed.
The internal conflict in Bulgaria continued to 1280 when Ivailo had in turn to flee to the Mongols and George I Terter ascended to the throne.