He conquered the land between the Severn and the Wye, and his dyke from the Dee to the Severn, and the Wye, marked the new limits of the Welsh and English borders; while his laws codified the customs of Mercia, as those of AEthelberht and Ine had done with the customs of Kent and Wessex.

He set up for awhile an archbishopric at Lichfield, which seems to mark his determination to erect Mercia into a sovereign power.

He also founded the great monastery of St.Alban's, and is said to have established the English college at Rome, though another account attributes it to Ine, the West Saxon.

East Anglia, Kent, Essex, and Sussex all acknowledged his supremacy.

Karl the Great was then reviving the Roman Empire in its Germanic form, and Offa ventured to correspond with the Frank emperor as an equal.