The supremacy that had once been in the hands of the Northumbrians now passed away to the kings of Mercia, the largest and most central of the English kingdoms. Three great kings of that realm, Aethelred, Aethelbald and Offa, whose reigns occupied almost the whole of the period from 675 to 796, were all in their day reckoned as supreme lords of England. The rulers of East Anglia, Essex, and Kent were counted as their vassals, and they deprived Wessex of its dominions north of the Thames, and Northumbria of all that it had held south of the Trent and the Ribble. Offa pushed his boundary far to the west, into the lands of the Welsh; and, after conquering the valleys of the Wye and the upper Severn, drew a great dyke from sea to sea, reaching from near Chester on the north to Chepstow on the south; it marked the boundary between the English and the Cymry for three hundred years. Offa was the greatest king whom England had yet seen, and corresponded on equal terms with Charles the Great, the famous King of the Franks, who was his firm friend and ally (757-796). [Sidenote: =Decline of Mercia.=] Nevertheless, after Offa's day the sceptre passed away from Mercia, and his successors saw their vassal kings rebel and disown the Mercian allegiance. To maintain subject states in obedience was always a very hard task for the old English kings, because they had no standing armies, and no system of fortification. When a neighbouring realm was overrun by the tumultuary army of a victorious king, he had to be satisfied with the homage of its people, because he could not build fortresses in it, or leave a standing force to hold it down. The only way of keeping a conquest was to colonize it, as was done with the lands taken from the Welsh; but the English kings shrank from evicting their own kinsfolk, and seldom or never employed this device against them. Hence it always happened that, when a great king died, his vassals at once rebelled, and unless his successor was a man of ability he was unable to reconquer them. [Sidenote: =Supremacy of Wessex.--Ecgbert, 800-836.=] From Mercia the primacy among the English states passed to Wessex, a state which had hitherto kept much to itself, and had busied itself in conquering land from the Welsh of Damnonia, rather than in striving with its English neighbours for the supremacy in mid-Britain. Wessex, indeed, had lost to the Mercians all its territory north of the Thames, and was now a purely south-country state. Its borders reached to the Tamar and the Cornish moors, since the days when Taunton in 710 and Exeter in 705 had fallen into the hands of its kings.