According to Porphyrogenitus, the Bulgars wanted to continue their conquest of the Slav lands and force the Serbs into subjugation. Presian I (r. 836–852) launched an invasion into Serbian territory in 839, which led to a war that lasted for three years, in which the Serbs were victorious; the heavily defeated Presian lost a large number of his men, made no territorial gains, and was driven out by Vlastimir's army. The Serbs held out in their easily defensible forests and gorges, and knew how to fight in the hills. The war ended with the death of Theophilos in 842, which released Vlastimir from his obligations to the Empire.