Canute was the
son of
Sweyn Forkbeard, King of
Denmark, who had conquered
England during the reign of
Ethelred the Unready. When Swyen died in
1014, Ethelred came back to the throne, and when Ethelred died in
1016, his son
Edmund II Ironside succeeded him. Canute, however, decided he wanted to rule England as his father had, and fought Edmund. The two eventually agreed to split the kingdom, but Edmund died within a year and Canute had England to rule alone. (He cemented his power by killing off as many other possible heirs as he could, such as one of Ethelred's younger sons; others fled the country.) He also conquered
Norway in
1030, giving him quite an
empire.
The version of the legend given in my sources is that Canute's advisors were trying to flatter him into thinking he was all-powerful, and he went to prove them wrong by commanding the tide to stop coming in, knowing he would fail. He then told them to realize "how empty and worthless is the power of kings." (This story is sometimes used as a Christian parable; some versions say Canute placed his crown on a statue of Christ after the tide came in.)
Canute died in 1035, probably aged around 40. His son Harold I Harefoot succeeded him to the throne of England (though not Denmark).