Complex sentence: In fact they also did not expect to achieve success so quickly and at first did not chase them but when the Bulgarians were certain that the enemy is fleeing in panic their heavy cavalry rushed after the Byzantines.
Split into simpler sentences: In fact they also did not expect to achieve success so quickly and at first did not chase them. But when the Bulgarians were certain that the enemy is fleeing in panic their heavy cavalry rushed after the Byzantines.