In 680 the Bulgars settled on the Balkans and allied with the more numerous Slavs living in the region, forming an independent and well organized political entity, the First Bulgarian Empire in 681. The major expansion of Bulgaria began in the first half of the 9th century when they attacked Constantinople and conquered numerous Slavic tribes on the Balkans (Guduscani, doubtful, and Timočani, which fled to the Frankish controlled areas in the west, Praedenecenti in c. 825, Merehani - all of which disappeared from history afterwards), so as the remnants of the Avars in the Pannonian plain. By acquiring the Morava Valley and Belgrade, they came in contact with the Serbs. Because of the ensuing Bulgarian-Serbian relations, the reign of prince Vlastimir can be determined with greater certainty. According to the "DAI", Presian I, Khan of Bulgaria, attacked the Serbs during his reign (Bulgar–Serb War (839–842)) in order to "subdue them". Unlike the tribes on the east, Vlastimir decided to stand the ground and not to lead Serbs to the west. Aside from the Bulgarians, he was surrounded by the Franks on the entire north, west and southwest side, including the vassal Zachlumia. He tried to strengthen his position by connecting with the subordinated Travunija, marrying his daughter into Travunija's ruling family. However, after the three years-long warfare, Presian didn't gain any territories and, additionally, lost the majority of its army.