Practically masters of all that lay north of Thames, the "great army" next moved against Wessex, the only quarter where a vigorous resistance was still maintained against them, though its capital, Winchester, had been sacked in 864. Under two kings named Halfdan and Bacsceg, and six earls, they seized Reading and began to harry Berkshire, Surrey and Hampshire. King Æthelred, the third son of Æthelwulf, came out against them, with his young brother Alfred and all the levies of Wessex. In the year 871 these two gallant kinsmen fought no less than six pitched battles against the invaders. Some were victories--notably the fight of Ashdown, where Alfred first won his name as a soldier--but the English failed to capture the fortified camps of the vikings at Reading, and were finally beaten at Marten ("Maeretun") near Bedwyn, where Æthelred was mortally wounded.
He left young sons, but the men of Wessex crowned Alfred king, because they needed a grown man to lead them in their desperate campaigning. Yet his reign opened inauspiciously: defeated near Wilton, he offered in despair to pay the vikings to depart. He must have known, from the experience of Mercian, Northumbrian and Frankish kings, that such blackmail only bought a short respite, but the condition of his realm was such that even a moderate time for reorganization might prove valuable. The enemy had suffered so much in the "year of the six battles" that they held off for some space from Wessex, seeking easier prey on the continent and in northern England. In 874 they harried Mercia so cruelly that King Burgred fled in despair to Rome; the victors divided up his realm, taking the eastern half for themselves, and establishing in it a confederacy, whose jarls occupied the "five boroughs" of Stamford, Lincoln, Derby, Nottingham and Leicester. But the western half they handed over to "an unwise thegn named Ceolwulf," who bought for a short space the precarious title of king by paying great tribute.
Alfred employed the four years of peace, which he had bought in 871, in the endeavour to strengthen his realm against the inevitable return of the raiders. His wisdom was shown by the fact that he concentrated his attention on the one device which must evidently prove effective for defence, if only he were given time to perfect it--the building of a national navy. He began to lay down galleys and "long ships," and hired "pirates"--renegade vikings no doubt--to train crews for him and to teach his men seamanship. The scheme, however, was only partly completed when in 876 three Danish kings entered Wessex and resumed the war. But Alfred blockaded them first in Wareham and then in Exeter. The fleet which was coming to carry them off, or to bring them reinforcements, fought an indecisive engagement with the English ships, and was wrecked immediately after on the cliffs of the Isle of Purbeck, where more than 100 galleys and all their crews perished. On hearing of this disaster the vikings in Exeter surrendered the place on being granted a free departure.