Vlastimir died c. 850 and Presijan in 852. Serbian throne was inherited by Vlastimir's sons, Mutimir, Strojimir and Gojnik. According to the conventional inheritance rules of the period, the state was probably administratively divided in three, but Mutimir held the "ruling right" as the eldest one. Presian's son and successor, Boris I of Bulgaria, decided to avenge his father, and attacked Serbia again, instigating the Bulgar–Serb War of 853, though the warfare is variously set by the historians in 854, 858, 863–864, 870 or even in the 880s. The war was also part of the larger skirmish in the region, which included the Bulgarian expansion in the direction of the central Danube Valley and the Byzantine efforts to weaken the Bulgarian-Frankish alliance against the Great Moravia and cultural turning of the Bulgarians to Rome. The Serbian army led by Mutimir and his brothers was again victorious, capturing the leader of the Bulgarian army, and Boris' son and heir, Vladimir of Bulgaria and 12 boyars. This pressured Boris to reluctantly agree to a peace treaty. As a guarantee that his son and the others will be freed, Boris asked for the Mutimir's sons to accompany the prisoners to the border. Mutimir sent his two younger sons, Bran and Stefan, while the eldest and heir to the throne, Pribislav, was precautionary kept at home. Pleased with the release of his son, Boris gave them "lush gifts", while the Serbian princes gave to Boris "two slaves, two falcons, two dogs, and 80 furs".