In the last quarter of the 8th century Bulgaria overcame the internal political crisis after the end of the rule of the Dulo. The khans Telerig and Kardam managed to consolidate the central authority and put an end of the quarrels among the nobility. The Bulgarians finally had the opportunity to intensify their campaigns in Slavic-populated Macedonia. In 789 they penetrated deep into the valley of the Struma river and heavily defeated the Byzantines, killing the strategos of Thrace Filites. 

Due to the rugged terrain the advancing Byzantine army broke its order. Taking advantage of that mistake, Kardam ordered a counterattack which brought the Bulgarians a great success. The Bulgarian cavalry went round the Byzantines and cut their way back to their fortified camp and the fortress of Marcellae. The Bulgarians took the supplies, the treasury and the tent of the emperor. They chased Constantine VI to Constantinople, killing a great number of soldiers. Many Byzantine commanders and officers perished in the battle. After the defeat, Constantine VI had to conclude peace with Kardam and had to pay tribute.