At the beginning of the ninth century, England was almost wholly under the control of the Anglo-Saxons. Mercia dominated southern England, but its supremacy came to an end in 825 when it was decisively defeated by Ecgberht at the Battle of Ellendun. The two kingdoms became allies, which was important in the resistance to Viking attacks. In 853, King Burgred of Mercia requested West Saxon help to suppress a Welsh rebellion, and Æthelwulf led a West Saxon contingent in a successful joint campaign. In the same year Burgred married Æthelwulf's daughter, Æthelswith.