Under Æthelfrith, Eadwine, and Oswiu, Northumbria had been the chief power in England. But the eighth century is taken up with the greatness of Mercia. Ecgfrith, the last great king of Northumbria, whose over-lordship extended over the Picts of Galloway and the Cumbrians of Strathclyde, endeavoured to carry his conquests beyond the Forth, and annex the free land lying to the north of the old Roman line. He was defeated and slain, and with him fell the supremacy of Northumbria. Mercia, which already, under Penda and Wulfhere, had risen to the second place, now assumed the first position among the Teutonic kingdoms. Unfortunately we know little of the period of Mercian supremacy. The West Saxon chronicle contains few notices of the rival state, and we are thrown for information chiefly on the second-hand Latin historians of the twelfth century. Æthelbald, the first powerful Mercian king (716-755), "ravaged the land of the Northumbrians," and made Wessex acknowledge his supremacy. By this time all the minor kingdoms had practically become subject to the three great powers, though still retaining their native princes: and Wessex, Mercia, and Northumbria shared between them, as suzerains, the whole of Teutonic Britain. The meagre annals of the Chronicle, upon which alone (with the Charters and Latin writers of later date) we rest after the death of Bæda, show us a chaotic list of wars and battles between these three great powers themselves, or between them and their vassals, or with the Welsh and Devonians. Æthelbald was succeeded, after a short interval, by Offa, whose reign of nearly forty years (758-796), is the first settled period in English history. Offa ruled over the subject princes with rigour, and seems to have made his power really felt. He drove the Prince of Powys from Shrewsbury, and carried his ravages into the heart of Wales. 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 Æthelberht 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. The possession of London, now a Mercian city, gave Offa an interest in continental affairs; and the growth of trade is marked by the fact that when a quarrel arose between them, they formally closed the ports of their respective kingdoms against each other's subjects.