Among the first to hurry to the court of William, as soon as Harold's accession to the throne was known, was Tostig, in whose mind the refusal of Harold to embark in a civil war for his sake, and to force him upon the people of Northumbria in spite of their detestation of him, was an injury not to be forgiven. The fact that Tostig was ready thus to sacrifice England to his own private quarrel showed a baseness and recklessness that could hardly be expected from his early career. William naturally accepted the alliance, received Tostig's oath of allegiance, and aided him in fitting out a number of ships manned by Norman and Flemish adventurers. Evading the watch kept by the English fleet they crossed the sea, landed, and plundered and ravaged a considerable extent of country, and then retired, Tostig being enraged that William of Normandy was unwilling to send an expedition to act in concert with him until the whole of his plans were prepared and his great army ready for sea.

Normandy indeed had been converted into a vast camp. In every port great numbers of workmen laboured night and day building ships, for Normandy had ceased to be a naval power, and its shipping was utterly insufficient to carry the great army across. Tostig, impatient and hasty, thought no more of the oath of allegiance that he had sworn to William. Driven from Yorkshire by the forces of the northern earls he sailed to Scotland, where he was welcomed by King Malcolm, both as a sworn brother and as the enemy of England. From Scotland he entered into negotiations with Harold Hardrada of Norway. This warlike monarch was in a fit mood to listen to his advances; he had for years been engaged in a struggle with Denmark, which he had ineffectually attempted to conquer, and had at last been forced to conclude a treaty of peace with Sweyn, its king.