With the succession of Khan Krum on the Bulgarian throne in 803 began the last and decisive part of the prolonged succession of Byzantine–Bulgarian conflicts which started in 756. For a little over a decade until his death in 814, Krum achieved significant successes over the Byzantine Empire, seizing the important city of Serdica in 809 and decisively defeating the Byzantine armies at the Varbitsa pass and at Versinikia. His son and successor Omurtag tried to continue Krum's aggressive policy but his campaign in 814 was stopped by the Byzantines. As both countries were exhausted by the sustained military effort, peace negotiations began.