From Wikipedia

In Search of Offa

Offa was the King of Mercia from 757 until his death in July 796. The son of Thingfrith and a descendant of Eowa, Offa came to the throne after a period of civil war following the assassination of Æthelbald after defeating the other claimant Beornred. In the early years of Offa's reign it is likely that he consolidated his control of midland peoples such as the Hwicce and the Magonsæte. Taking advantage of instability in the kingdom of Kent to establish himself as overlord, Offa was also in control of Sussex by 771, though his authority did not remain unchallenged in either territory. In the 780s he extended Mercian supremacy over most of southern England, allying with Beorhtric of Wessex, who married Offa's daughter Eadburh, and regained complete control of the southeast. He also became the overlord of East Anglia, and had King Æthelberht II of East Anglia beheaded in 794, perhaps for rebelling against him.