William of Normandy believed he should be king of England because his friend and first cousin once removed, Edward the Confessor, who was the childless king of England from 1042 until his death in 1066, promised William that he would be his successor. When Edward died, however, the deceased king's brother-in-law, Harold, assumed the throne instead. In response to what he felt was an act of betrayal, William decided to take the throne by force and launched a military campaign against the new monarch.
William's campaign was successful and King Harold was defeated and killed on October 14, 1066 at the Battle of Hastings 10 months after having assumed the throne. William, who was also known as William the Conqueror, was made King of England at a coronation ceremony that took place at Westminster Abbey, London on Christmas Day, 1066. He was the first Norman king of England and he reigned until his death in 1087.