In his work of organization, in his increase of bishoprics, in his arrangement of dioceses, and the way in which he grouped them round the see of Canterbury, in his national synods and ecclesiastical canons, Theodore was unconsciously doing a political work. The old divisions of kingdoms and tribes about him, divisions which had sprung for the most part from mere accidents of the conquest, were fast breaking down. The smaller states were by this time practically absorbed by the three larger ones, and of these three Mercia and Wessex had for a time bowed to the overlordship of Northumbria. The tendency to national unity which was to characterize the new England had thus already declared itself; but the policy of Theodore clothed with a sacred form and surrounded with divine sanctions a unity which as yet rested on no basis but the sword. The single throne of the one primate at Canterbury accustomed men's minds to the thought of a single throne for their one temporal overlord at York, or, as in later days, at Lichfield or at Winchester. The regular subordination of priest to bishop, of bishop to primate, in the administration of the Church, supplied a mould on which the civil organization of the state quietly shaped itself.
Above all, the councils gathered by Theodore were the first of all national gatherings for general legislation. It was at a much later time that the Wise Men of Wessex, or Northumbria, or Mercia, learned to come together in the Witenagemot of all England. It was the ecclesiastical synods which by their example led the way to our national parliament, as it was the canons enacted in such synods which led the way to a national system of law. But if the movement towards national unity was furthered by the centralizing tendencies of the Church, it was as yet hindered by the upgrowth of a great rival power to contest the supremacy with Northumbria. Mercia, as we have seen, had recovered from the absolute subjection in which it was left after Penda's fall by shaking off the supremacy of Oswiu, and by choosing Wulfhere for its king. Wulfhere was a vigorous and active ruler, and the peaceful reign of Oswiu left him free to build up again during the sixteen years of his rule the power which had been lost at Penda's death. Penda's realm in Central Britain was quickly restored, and Wulfhere's dominion extended even over the Severn and embraced the lower valley of the Wye. He had even more than his father's success.
After a great victory in 661 over the West-Saxons, his ravages were carried into the heart of Wessex, and the valley of the Thames opened to his army. To the eastward, the East-Saxons and London came to own his supremacy; while southward he pushed across the river over Surrey. In the same year, 661, Sussex, perhaps in dread of the West-Saxons, found protection in accepting Wulfhere's overlordship, and its king was rewarded by a gift of two outlying settlements of the Jutes, the Isle of Wight and the lands of the Meon-wara along the Southampton Water, which we must suppose had been reduced by Msrcian arms. The Mercian supremacy which thus reached from the Humber to the Channel and stretched westward to the Wye was the main political fact in Britain when Theodore landed on its shores. In fact, with the death of Oswiu in 670 all effort was finally abandoned by Northumbria to crush the rival states in Central or Southern Britain.
The industrial progress of the Mercian kingdom went hand in hand with its military advance. The forests of its western border, the marshes of its eastern coast, were being cleared and drained by monastic colonies, whose success shows the hold which Christianity had now gained over its people. Heathenism indeed still held its own in the western woodlands; we may perhaps see Woden-worshipping miners at Alcester in the daemons of the legend of Bishop Ecgwine of Worcester, who drowned the preacher's voice with the din of their hammers. But in spite of their hammers Ecgwine's preaching left one lasting mark behind it. The bishop heard how a swineherd, coming out from the forest depths on a sunny glade, saw forms which were possibly those of the Three Fair Women of the old German mythology, seated round a mystic bush, and singing their unearthly song. In his fancy the fair women transformed themselves into a vision of the Mother of Christ; and the silent glade soon became the site of an abbey dedicated to her, and of a town which sprang up under its shelter - the Evesham which was to be hallowed in after time by the fall of Earl Simon of Leicester. Wilder even than the western woodland was the desolate fen-country on the eastern border of the kingdom, stretching from the "Holland," the sunk, hollow land of Lincolnshire, to the channel of the Ouse, a wilderness of shallow waters and reedy islets wrapped in its own dark mist-veil and tenanted only by flocks of screaming wild-fowl. Here through the liberality of King Wulfhere rose the abbey of Medeshamstead, our later Peterborough. On its northern border a hermit, Botulf, founded a little house which as ages went by became our Botulf's town or Boston. The abbey of Ely was founded in the same wild fen-country by the Lady aethel-thryth, the wife of King Ecgfrith, who in the year 670 succeeded Oswiu on the throne of Northumbria. Here, too, Guthlac, a youth of the royal race of Mercia, sought a refuge from the world in the solitude of Crowland, and so great was the reverence he won, that only two years had passed since his death when the stately abbey of Crowland rose over his tomb.
Earth was brought in boats to form a site; the buildings rested on oaken piles driven into the marsh, a stone church replaced the hermit's cell, and the toil of the new brotherhood changed the pools around them into fertile meadow-land.
But while Mercia was building up its dominion in Mid-Britain, Northumbria was far from having sunk from its old renown either in government or war. Ecgfrith had succeeded his father Oswiu in 670, and made no effort to reverse his policy, or attempt to build up again a supremacy over the states of southern Britain. His ambition turned rather to conquests over the Briton than to victories over his fellow Lnglishmen. The war between Briton and Englishman, which had languished since the battle of Chester, had been revived some twenty years before by an advance of the West-Saxons to the south-west. Unable to save the possessions of Wessex in the Severn valley and on the Cotswolds from the grasp of Penda, the West-Saxon king, Cenwealh, seized the moment when Mercia was absorbed in the last struggle of Penda against Northumbria to seek for compensation in an attack on his Welsh neighbours. A victory at Bradford on the Avon enabled him to overrun the country north of Mendip which had till then been held by the Britons; and a second campaign in 658, which ended in a victory on the skirts of the great forest that covered Somerset to the east, settled the West-Saxons as conquerors round the sources of the Parret. It may have been the example of the West-Saxons which spurred Ecgfrith to enlarge the bounds of his kingdom by a series of attacks upon his British neighbours in the west.