The cause of the sudden outpouring of the Scandinavian deluge upon the lands of Christendom at this particular date is one of the puzzles of history. So far as memory ran, the peoples beyond the North Sea had been seafaring races addicted to piracy. Even Tacitus mentions their fleets. Yet since the 5th century they had been restricting their operations to their own shores, and are barely heard of in the chronicles of their southern neighbours. It seems most probable that the actual cause of their sudden activity was the conquest of the Saxons by Charles the Great, and his subsequent advance into the peninsula of Denmark. The emperor seemed to be threatening the independence of the North, and in terror and resentment the Scandinavian peoples turned first to strike at the encroaching Frank, and soon after to assail the other Christian kingdoms which lay behind, or on the flank of, the Empire. But their offensive action proved so successful and so profitable that, after a short time, the whole manhood of Denmark and Norway took to the pirate life. Never since history first began to be recorded was there such a supreme example of the potentialities of sea-power. Civilized Europe had been caught at a moment when it was completely destitute of a war-navy; the Franks had never been maritime in their tastes, the English seemed to have forgotten their ancient seafaring habits. Though their ancestors had been pirates as fierce as the vikings of the 9th century, and though some of their later kings had led naval armaments--Edwin had annexed for a moment Man and Anglesea, and Ecgfrith had cruelly ravaged part of Ireland--yet by the year 800 they appear to have ceased to be a seafaring race. Perhaps the long predominance of Mercia, an essentially inland state, had something to do with the fact. At any rate England was as helpless as the Empire when first the Danish and Norwegian galleys began to cross the North Sea, and to beat down both sides of Britain seeking for prey. The number of the invaders was not at first very great; their fleets were not national armaments gathered by great kings, but squadrons of a few vessels collected by some active and enterprising adventurer. Their original tactics were merely to land suddenly near some thriving seaport, or rich monastery, to sack it, and to take to the water again before the local militia could turn out in force against them. But such raids proved so profitable that the vikings soon began to take greater things in hand; they began to ally themselves in confederacies: two, six or a dozen "sea-kings" would join their forces for something more than a desultory raid. With fifty or a hundred ships they would fall upon some unhappy region, harry it for many miles inland, and offer battle to the landsfolk unless the latter came out in overpowering force. And as their crews were trained warriors chosen for their high spirit, contending with a raw militia fresh from the plough, they were generally successful.