After the siege of Lovech in 1187, the Byzantine Emperor Isaac II Angelos was forced to conclude a truce, thus de facto recognizing the independence of Bulgaria. Until 1189, both sides observed the truce. The Bulgarians used this time to further organize their administration and military. When the soldiers of the Third Crusade reached the Bulgarian lands at Niš, Asen and Peter offered to help the Emperor of the Holy Roman Empire, Frederick I Barbarosa, with a force of 40,000 against the Byzantines. However, the relations between the Crusaders and the Byzantines smoothed, and the Bulgarian proposal was evaded.

The Byzantines prepared a third campaign to avenge the Bulgarian actions. Like the previous two invasions, they managed to overcome the passes of the Balkan mountains. They made a bluff indicating that they would pass near the sea by Pomorie, but instead headed west and passed through the Rishki Pass to Preslav. The Byzantine army next marched westwards to besiege the capital at Tarnovo. At the same time, the Byzantine fleet reached the Danube in order to bar the way of Cuman auxiliaries from the northern Bulgarian territories.

The siege of Tarnovo was unsuccessful. The defense of the city was led by Asen himself and the morale of his troops was very high. The Byzantine morale, on the other hand, was quite low for several reasons: the lack of any military success, heavy casualties and particularly the fact that the soldiers' pay was in arrears. This was used by Asen, who sent an agent in the guise of a deserter to the Byzantine camp. The man told Isaac II that, despite the efforts of the Byzantine navy, an enormous Cuman army had passed the river Danube and was heading towards Tarnovo to relive the siege. The Byzantine Emperor panicked and immediately called for a retreat through the nearest pass.