Why did William I claim that he should inherit the English crown?

William I’s claim to the English throne was primarily based on his assertion that Edward the Confessor, who was childless, had designated him as his successor.123

Historical accounts, particularly those from Norman sources, suggest that Edward promised the throne to William as early as 1051. William further bolstered this claim by asserting that Harold II had sworn a sacred oath in 1064 to support his right to the crown. When Harold was accepted as king by the English magnates following Edward’s death in 1066, William viewed Harold’s accession as a violation of that oath.1243

William used this alleged perjury as a central justification for his invasion, successfully framing his military campaign as a righteous cause to reclaim his promised inheritance. To solidify his position, he also secured the blessing of Pope Alexander II, who supported the invasion partly due to the perceived illegitimacy of the English church leadership at the time.123