In five 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 change could only have been made possible by the temper of the conquered kingdoms.  To them the conquest was simply their transfer from one overlord to another, and it may be that in all there were men who preferred the overlordship of the Northman to the overlordship of the West-Saxon.  But the loss of the subject kingdoms left Wessex face to face with the invaders.  The time had now come for it to fight, not for supremacy, but for life.  As yet the land 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 pirates no sooner pushed up Thames to Reading in 871 than the West-Saxons, attacked on their own soil, turned fiercely at bay.  A desperate attack drove the northmen from Ashdown on the heights that overlook the Vale of White Horse, but their camp in the tongue of land between the Kennet and Thames proved impregnable.  AEthelred died in the midst of the struggle, and his brother AElfred, who now became king, bought the withdrawal of the pirates and a few years’ breathing-space for his realm.  It was easy for the quick eye of AElfred to see that the northmen had withdrawn simply with the view of gaining firmer footing for a new attack; three years indeed had hardly passed before Mercia was invaded and its under-king driven over sea to make place for a tributary of the invaders.  From Repton half their host marched northwards to the Tyne, while Guthrum led the rest to Cambridge to prepare for their next year’s attack on Wessex.  In 876 his fleet appeared before Wareham, and in spite of a treaty bought by AElfred, the northmen threw themselves into Exeter.  Their presence there was likely to stir a rising of the Welsh, and through the winter AElfred girded himself for this new peril.  At break of spring his army closed round the town, a hired fleet cruised off the coast to guard against rescue, and the defeat of their fellows at Wareham in an attempt to relieve them drove the pirates to surrender.  They swore to leave Wessex and withdrew to Gloucester.  But AElfred had hardly disbanded his troops when his enemies, roused by the arrival of fresh hordes eager for plunder, reappeared at Chippenham, and in the opening of 878 marched ravaging over the land.  The surprise of Wessex 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, a position from which he could watch closely the movements of his foes.  But with the first burst of spring he called the thegns of Somerset to his standard, and still gathering troops as he moved marched through Wiltshire on the northmen.  He found their host at Edington, defeated it in a great