After a short interval of unrest the Mercian throne was filled by Offa, a distant relation of Ethelbald, who reigned for nearly forty years (757–96), and who in some ways seems to deserve the title of the greatest of Mercian kings. The everlasting contest with Wessex was renewed, and Offa’s victory at Bensington in Oxfordshire (779) did something towards obliterating the disgrace of Burford and probably gave what is now the county of Oxford to the middle kingdom. From various causes Offa had now acquired so great a predominance that he was able to carry into effect a change in the ecclesiastical geography of England which was little less than a revolution. This was the creation of a new archbishopric for the Midlands. We may imagine that he reasoned in this wise: “Northumbria has now its archbishopric at York. The archbishop of Canterbury is too much overshadowed by the greatness of my rival of Wessex. Why should not I, the most powerful king in Britain, have an archbishop of my own here in Mercia?” This reasoning prevailed. In 787 a synod, ever after known as “the contentious synod,” was held at Chelsea, and thereat, we are told, seven out of the twelve dioceses of the southern province were placed under the archbishop of Lichfield, being rent away from their dependence on Canterbury. The meaning of this change is obvious. There were now three great English kingdoms: Northumbria, Wessex and Mercia, and three corresponding archbishoprics, York, Lichfield and Canterbury. The Thames was the boundary between the central and southern provinces, except that Essex with Middlesex was included in the latter. East Anglia was evidently, in ecclesiastical matters as well as in things political, subject to Mercia, a fact which accounts for the abrupt entry in the Chronicle for 792[113] (794): “Offa, King of the Mercians, ordered the head of Ethelbert, King [of the East Angles], to be struck off”. The new ecclesiastical arrangement lasted for only sixteen years. In 803 Offa’s successor Cenwulf voluntarily restored all the metropolitan rights of the see of Canterbury.