But in 552, the English Chronicle tells us, Cynric, the West Saxon king, crossed the downs behind Winchester, and descended upon the dale at Salisbury.

The Roman town occupied the square hill-fort of Old Sarum, and there Cynric put the Welsh to flight and took the stronghold by storm.
The road was thus opened in the rear to the upper waters of the Thames (impassable before because of the Roman population of London), as well as towards the valley of the Bath Avon.

Four years later Cynric and his son Ceawlin once more advanced as far as Barbury hill-fort, probably on a mere plundering raid.

But in 571 Cuthwulf, brother of Ceawlin, again marched northward, and "fought against the Welsh at Bedford, and took four towns, Lenbury (or Leighton Buzzard), Aylesbury, Bensington (near Dorchester in Oxfordshire), and Ensham." Thus the West Saxons overran the whole upper valley of the Thames from Berkshire to above Oxford, and formed a junction with the Middle Saxons to the north of London; while eastward they spread as far as the northern boundaries of Essex.

In 577 the same intruders made a still more important move.