Answers (1)
Legend has it that Harold supported William's claim to the throne as legitamte. However, when King Edward the Confessor died in January 1066, Harold rushed to London and had himself crowned King. As a result, William invaded England and defeated Harold at Hastings.
The legend is that Harold promised to support Williams claim to the throne over his own, but when both the Witenagemot and King Edward himself (who was also supposed to have promised the throne to William) selected Harold to succeed Edward, he accepted it. This was a betrayal in William's eyes, and it committed the two kingdoms to War.