After almost twenty years of conflict, the rulers of Norway and Denmark, Harald Hardrada and Sweyn Estrithson made peace. This allowed Harald to turn his attentions to England where he believed he had a right to the throne. In 1038 Magnus of Norway and Harthacnut of Denmark had signed a treaty that specified if either died without an heir his kingdom must pass to the other. When Harthacnut died childless in 1042 Magnus claimed both Denmark and England as his own. It was this claim that Harald was basing his own claim upon.

An attack by a Danish invasion fleet led by Sweyn Estrithson of Denmark was fought off by a garrison stationed at a castle at Sandwich. Sandwich was an important port on the south coast. The location has now been found and the site is being investigated.

An army sent by Sweyn of Denmark landed in the north and captured York. Local rebels joined the Danes and attacked the two castles within the city.

Upon the death of Sweyn the King of Denmark a war of succession followed between his eldest sons. Harold became king and reigned until 1081.