The king of England in the latter half of the Tenth Century, Æthelred the Deuce, (known to history as "Ethelred the Unready",) was getting pretty tired of Viking raids all up and down the English coast. He was especially annoyed at reports that the raiders were being given aid and comfort by the undocumented Danish aliens who had been settling in the eastern half of the country.
Therefore he launched a cunning plan: In order to cut down on the amount of assistance they could give the seagoing freelance tax men, he would have all the Dane settlers in England put to death. Every man jack of them. Every woman jack and child jack, too. The massacres were carried out on St. Brice's Day, November 13th of 1002AD.
Unfortunately for Æthelred, one of the woman jacks he had killed had a brother, and her brother was currently working as a king in Denmark, going by the rather butch handle of "Sweyn Forkbeard". Sweyn saddled up his posse and after the requisite amount of burning and pillaging and bribing and betraying, Æthelred skipped town for Normandy ahead of the Viking horde and Sweyn was crowned king of England, too.
With two crowns under his belt, as it were, Sweyn only needed to conquer three more countries and he'd get the sixth free, but he died before he could get any more punches on his card, leaving his son Canute to deal with Britain and Æthelred.