Accepting he could not conquer Denmark, Harald Hardrada switched attention to England; his claim was based on a 1038 agreement between Magnus and its previous ruler, Harthacnut, who died childless in 1042. This stated if either died, the other would inherit their lands. Hardrada invaded northern England in early September, leading a fleet of more than 300 ships carrying perhaps 15,000 men. Hardrada's army was further augmented by the forces of Tostig, who supported the Norwegian king's bid for the throne.