After the death of Prince Vlastimir of Serbia in c. 850, his state was divided between his sons. Vlastimir and Presian, Boris' father, had fought each other in the Bulgar-Serb War (839–42), which resulted in a Serbian victory. Boris sought to avenge that defeat, an in 853 or 854, the Bulgar army led by Vladimir-Rasate, the son of Boris I, invaded Serbia, with the aim to replace the Byzantine overlordship on the Serbs. The Serbian army was led by Mutimir and his two brothers, who defeated the Bulgars, capturing Vladimir and 12 boyars. Boris I and Mutimir agreed on peace (and perhaps an alliance), and Mutimir sent his sons Pribislav and Stefan to the border to escort the prisoners, where they exchanged items as a sign of peace. Boris himself gave them "rich gifts", while he was given "two sl