The sons fought many wars on behalf of their father. Charles was mostly preoccupied with the Bretons, whose border he shared and who insurrected on at least two occasions and were easily put down. He also fought the Saxons on multiple occasions. In 805 and 806, he was sent into the Böhmerwald (modern Bohemia) to deal with the Slavs living there (Bohemian tribes, ancestors of the modern Czechs). He subjected them to Frankish authority and devastated the valley of the Elbe, forcing tribute from them. Pippin had to hold the Avar and Beneventan borders and fought the Slavs to his north. He was uniquely poised to fight the Byzantine Empire when that conflict arose after Charlemagne's imperial coronation and a Venetian rebellion. Finally, Louis was in charge of the Spanish March and fought the Duke of Benevento in southern Italy on at least one occasion. He took Barcelona in a great siege in 797.