The disastrous rule of the last Comnenian emperor Andronikos I (r. 1183–85) worsened the situation of the Bulgarian peasantry and nobility. The first act of his successor Isaac II Angelos was to impose an extra tax to finance his wedding. In 1185, two aristocrat brothers from Tarnovo, Theodore and Asen, asked the emperor to enlist them into the army and grant them land, but Isaac II declined and slapped Asen across the face.

Upon their return to Tarnovo, the brothers commissioned the construction of a church dedicated to Saint Demetrius of Salonica. They showed the populace a celebrated icon of the saint, who they claimed had left Salonica to support the Bulgarian cause and called for a rebellion. That act had the desired effect on the religious population, who enthusiastically engaged in a rebellion against the Byzantines. Theodore, the elder brother, was crowned Emperor of Bulgaria under the name Peter IV. Almost all of Bulgaria to the north of the Balkan Mountains—the region known as Moesia—immediately joined the rebels, who also secured the assistance of the Cumans, a Turkic tribe inhabiting lands north of the Danube river. The Cumans soon became an important part of the Bulgarian army, playing a major role in the successes that followed. As soon as the rebellion broke out, Peter IV attempted to seize the old capital of Preslav but failed; he declared Tarnovo the capital of Bulgaria.