but his armies did not attack, and he received a small area in Strandzha to the southeast. The peace treaty was not signed, however, although both states exchanged temporary delegations. In 854 the Moravian Prince Rastislav persuaded Boris I to help him against East Francia. According to some sources, some Franks bribed the Bulgarian monarch to attack Louis the German. The Bulgarian-Slav campaign was a disaster, and Louis scored a great victory and invaded Bulgaria. At the same time the Croats waged a war against the Bulgarians. Both peoples had coexisted peacefully up to that time, suggesting that the Croats were paid by Louis to attack Bulgaria and distract Boris' attention from his alliance with Great Moravia. Kanasubigi Boris could not achieve any success, and both sides exchanged gifts and settled for peace. As a result of the military actions in 855, the peace between Bulgaria and Eastern Francia was restored, and Rastislav was forced to fight against Louis alone. In the meantime, a conflict between the Byzantines and Bulgarians had started in 855–856, and Boris, distracted by his conflict with Louis, lost Philippopolis (Plovdiv), the region of Zagora, and the ports around the Gulf of Burgas on the Black Sea to the Byzantine army led by Michael III and the caesar Bardas. Serbia After the death of Knez Vlastimir of Serbia circa 850, his state was divided between his sons. Vlastimir and Boris' father had fought against each other in the Bulgarian-Serbian War of 839–842, which resulted in a Serbian victory, and Boris sought to avenge that defeat. In 853 or 854, the Bulgarian army led by Vladimir-Rasate, the son of Boris I, invaded Serbia, with the aim of replacing the Byzantine overlordship over the Serbs. The Serbian army was led by Mutimir and his two brothers; they defeated the Bulgarians, capturing Vladimir and 12 boyars. Boris I and Mutimir agreed to 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 slaves, two falcons, two dogs, and 80 furs". An internal conflict among the Serbian brothers resulted in Mutimir banishing the two younger brothers to the Bulgarian court. Mutimir, however, kept a nephew, Petar, at his court for political reasons. The reason for the feud is not known,