If the odds were too great they could always retire to their ships, put to sea, and resume their predatory operations on some other coast three hundred miles away. As long as their enemies were unprovided with a navy they were safe from pursuit and annihilation. The only chance against them was that, if caught too far from the base-fort where they had run their galleys ashore, they might find their communication with the sea cut off, and be forced to fight for their lives surrounded by an infuriated countryside. But in the earlier years of their struggles with Christendom the vikings seldom suffered a complete disaster; they were often beaten but seldom annihilated. Ere long they grew so bold that they would stay ashore for months, braving the forces of a whole kingdom, and sheltering themselves in great palisaded camps on peninsulas or islands when the enemy pressed them too hard. On well-guarded strongholds like Thanet or Sheppey in England, Noirmoutier at the Loire mouth, or the Isle of Walcheren, they defied the local magnates to evict them. Finally they took to wintering on the coast of England or the Empire, a preliminary to actual settlement and conquest. (See VIKING.) Progress of Danish conquest. King Ecgbert died long ere the invaders had reached this stage of insolence. Æthelwulf, his weak and kindly son, would undoubtedly have lost the titular supremacy of Wessex over the other English kingdoms if there had been in Mercia or Northumbria a strong king with leisure to concentrate his thoughts on domestic wars. But the vikings were now showering such blows on the northern states that their unhappy monarchs could think of nothing but self-defence. They slew Redulf--king of Northumbria--in 844, took London in 851, despite all the efforts of Burgred of Mercia, and forced that sovereign to make repeated appeals for help to Æthelwulf as his overlord. For though Wessex had its full share of Danish attacks it met them with a vigour that was not seen in the other realms. The defence was often, if not always, successful; and once at least (at Aclea in 851) Æthelwulf exterminated a whole Danish army with "the greatest slaughter among the heathen host that had been heard of down to that day," as the Anglo-Saxon chronicler is careful to record. But though he might ward off blows from his own realm, he was helpless to aid Mercia or East Anglia, and still more the distant Northumbria.