The petty squadrons which had till now harassed the coast of Britain made way for larger hosts than had as yet fallen on any country in the west; while raid and foray were replaced by the regular campaign of armies who marched to conquer, and whose aim was to settle on the land they won. In 866 the Danes landed in East Anglia, and marched in the next spring across the Humber upon York. Civil strife as usual distracted the energies of Northumbria. Its subject-crown was disputed by two claimants, and when they united to meet this common danger both fell in the same defeat before the walls of their capital. Northumbria at once submitted to the Danes, and Mercia was only saved by a hasty march of King aethelred to its aid. But the Peace of Nottingham, by which aethelred rescued Mercia in 868, left the Danes free to turn to the rich spoil of the great abbeys of the Fen. Peterborough, Crow-land, Ely, went up in flames, and their monks fled or were slain among the ruins. From thence they struck suddenly for East Anglia itself, whose king, Eadmund, brought prisoner before the Danish leaders, was bound to a tree and shot to death with arrows.
His martyrdom by the heathen made him the St. Sebastian of English legend; in later days his figure gleamed from the pictured windows of church after church along the eastern coast, and the stately abbey of St. Edmunds-bury rose over his relics. With Eadmund ended the line of East Anglian under-kings, for his kingdom was not only conquered, but ten years later it was divided among the soldiers of a Danish host, whose leader, Guthrum, assumed its crown. How great was the terror stirred by these successive victories was shown in the action of Mercia, which, though it was as yet still spared from actual conquest, crouched in terror before the Danes, acknowledged them in 870 as its overlords, and paid them tribute.
In four years the work of Ecgberht had been undone, and England north of the Thames had been torn from the overlordship of Wessex. So rapid a conquest as the Danish conquests of Northumbria, Mercia, and East Anglia, had only been made possible by the temper of these kingdoms themselves. To them the conquest was simply their transfer from one overlord to another, and it would seem as if they preferred the lordship of the Dane to the overlordship of the West-Saxon. It was another sign of the enormous difficulty of welding these kingdoms together into a single people. The time had now come for Wessex to fight, not for supremacy, but for life. As yet it seemed paralyzed by terror. With the exception of his one march on Nottingham, King aethelred had done nothing to save his under-kingdoms from the wreck. But the Danes no sooner pushed up Thames to Reading than the West-Saxons, attacked on their own soil, turned fiercely at bay. The enemy penetrated indeed into the heart of Wessex as far as the heights that overlook the Vale of White Horse. A desperate battle drove them back from Ashdown; but their camp in the tongue of land between the Kennet and Thames proved impregnable, and fresh forces pushed up the Thames to join their fellows.
In the midst of the struggle aethelred died, and left his youngest brother aelfred to meet a fresh advance of the foe. They had already encamped at Wilton before the young king could meet them, and a series of defeats forced him to buy the withdrawal of the pirates and win a few years' breathing-space for his realm. It was easy for the quick eye of aelfred to see that the Danes had withdrawn simply with the view of gaining firmer footing for a new attack; indeed, three years had hardly passed before Mercia was invaded, and its under-king driven over sea to make place for a tributary of the Danes. From Repton half their host marched northwards to the Tyne, dividing a land where there was little left to plunder, colonizing and tilling it, while Guthrum led the rest into East Anglia to prepare for their next year's attack on Wessex. The greatness of the contest had now drawn to Britain the whole strength of the northmen; and it was with a host swollen by reinforcements from every quarter that Guthrum at last set sail for the south. In 876 the Danish fleet appeared before Wareham, and when a treaty with aefred won their withdrawal, they threw themselves into Exeter and allied themselves with the Welsh. Through the winter aefred girded himself for this new peril.
At break of spring his army closed round the town, while a hired fleet cruised off the coast to guard against rescue. The peril of their brethren in Exeter forced a part of the Danish host which had remained at Wareham to put to sea with the view of aiding them, but they were driven by a storm on the rocks of Swanage, and Exeter was at last starved into surrender, while the Danes again swore to leave Wessex.
They withdrew in fact to Gloucester, but aefred had hardly disbanded his troops when his enemies, roused by the arrival of fresh hordes eager for plunder, reappeared at Chippenham, and at the opening of 878 marched ravaging over the land. The surprise was complete, and for a month or two the general panic left no hope of resistance. aelfred, with his small band of followers, could only throw himself into a fort raised hastily in the isle of Athelney, among the marshes of the Parret. It was a position from which he could watch closely the movements of his foes, and with the first burst of spring he called the thegns of Somerset to his standard, and still gathering his troops as he moved, marched through Wiltshire on the Danes. He found their host at Edington, defeated it in a great battle, and after a siege of fourteen days forced them to surrender. Their leader, Guthrum, was baptized as a Christian and bound by a solemn peace or "frith " at Wedmore in Somerset. In form the Peace of Wedmore seemed indeed a surrender of the bulk of Britain to its invaders. All North-umbria, all East Anglia, the half of Central England was left subject to the northmen.