His first few years as king are not well recorded. It is very probable that he used this time to reorganize his army and his administration. Egbert formed an alliance with Wulfred, Archbishop of Canterbury. In 815 Egbert invaded Cornwall and brought it under his rule. He allowed sub-kings to rule it for him. In 825 the new king of Mercia, Beornwulf, invaded Wessex. Beornwulf had extended his authority over Kent, Essex and Middlesex. In what Frank Stenton called "one of the most decisive battles of Anglo-Saxon history" Egbert defeated Beornwulf. This was the end of the Mercian kings dominating England. Egbert sent his son Athelwulf with an army to overthrow Baldred, the Mercian sub-king. As a result, all of Kent, Surrey, Sussex and East Anglia submitted to Egbert. Beornwulf tried to recover East Anglia later the same year and was killed in the attempt. Kent, Surrey and Sussex remained a part of the kingdom of Wessex from this time on. Essex was later lost to the Danes. Egbert appointed his son Athelwulf as sub-king of Kent which included all the new territories of Wessex.