When Siward, Earl of Northumbria died in 1055, his only surviving son was still a child, and King Edward awarded the earldom to Harold’s inexperienced brother Tostig. Although the King was no fan of of the late Earl Godwine, there is evidence that Godwine’s third son was a favorite at court. Since he was half Danish, Tostig didn’t seem like a bad choice for the region inhabited by Norsemen, and he served as Earl of Northumbria for ten years before he had serious trouble.
Although he was accused of being overzealous in enforcing law and order, tempers did not rise to the boiling point until he imposed a new substantial tax burden – possibly to help pay for the Welsh campaign he had just waged with Harold in 1063. Suddenly, all the Northumbrian thegns united against him. On October 3, 1065, while Tostig was hunting with the King, the rebels descended on York and raided the treasury, killing two housecarls and more than 200 officials. They declared Tostig outlaw and sent for Morcar, younger son of Aelfgar who represented the most powerful rival of Godwine’s family (Morcar’s elder brother Edwin was already Earl of Mercia). Then they sent to the King to confirm their decision and rampaged their way south, gaining support along the way.
Harold Godwineson was chosen to mediate, and met the rebels at Northampton; he had the backing of the King and of Tostig, who had every reason to believe that he would get his earldom back one way or the other. Alas, Harold was in a big predicament. He soon learned that Tostig had lost all support in Northumbria; in fact, the only way the Northumbrians could be compelled to accept Tostig back was by force.
Still plundering the area around Northampton, the rebels sent Harold back to the King along with their own envoys, demanding the election of Morcar and outlawry of Tostig. Harold reluctantly complied, and advised the King against using military force. Everyone was shocked, and an irate Tostig accused Harold of complicity.
Although Edward initially sought to raise the fyrd against the northerners, his subjects had a horror of civil war – especially for a lost cause – and the King met so much resistance he was soon obliged to accept the rebels’ terms. He reluctantly sent Harold back with orders to depose Tostig and elect Morcar, pardoning the thegns and reinstituting the laws of Canute.
Swearing vengeance, Tostig went into voluntary exile and Edward’s health slipped into decline the following month, possibly in grief and shock at his loss of authority. The natural allegiance of Harold and Tostig was broken forever, and the next time they were to meet would be on the battlefield.