How did Harald III of Norway and Tostig threaten Harold II’s rule?

Harald III of Norway and Tostig, the exiled brother of King Harold II, posed a significant threat to the English crown by forming a military alliance to invade England in 1066.12

Tostig, who had been ejected from his earldom in Northumbria the previous year, sought to regain his position by supporting Harald III’s ambition to conquer the English throne. Together, they launched a major invasion, crossing the North Sea with a fleet of 300 ships and sailing up the Humber River.1

Their threat to Harold II’s rule manifested in several ways:
* Military Defeat: In September 1066, the invading forces defeated the Earls of Northumbria and Mercia at the Battle of Fulford, allowing them to occupy York.31
* Strategic Distraction: The invasion forced King Harold II to march his army rapidly to the north, leaving the southern coast of England vulnerable.14
* Resource Depletion: Because Harold II had to disband his southern army earlier in September due to a lack of supplies and the need for soldiers to return to the harvest, the northern conflict left the country open to the subsequent Norman invasion by William the Conqueror.4

Harold II ultimately defeated the alliance at the Battle of Stamford Bridge on September 25, 1066, where both Harald III and Tostig were killed.314