In the 1050s and early 1060s William became a contender for the throne of England, then held by his childless first cousin once removed Edward the Confessor.
There were other potential claimants, including the powerful English earl Harold Godwinson, who was named the next king by Edward on the latter's deathbed in January 1066.
William argued that Edward had previously promised the throne to him, and that Harold had sworn to support William's claim.
William built a large fleet and invaded England in September 1066, decisively defeating and killing Harold at the Battle of Hastings on 14Oct1066.
After further military efforts William was crowned king on Christmas Day 1066, in London. He made arrangements for the governance of England in early 1067 before returning to Normandy.
Several unsuccessful rebellions followed, but by 1075 William's hold on England was mostly secure, allowing him to spend the majority of the rest of his reign on the continent.