Canute the Great
He accompanied his father on Svein's invasion of England in 1013 and, when Svein died the following February, the army declared him king. However, the English had other ideas and recalled from exile Ethelred Ill-Counsel, from his exile in Normandy. Canute, aware that he needed to cement his succession in his father's principal kingdom, Denmark, returned there (though not before pointing out his displeasure to the English by savagely mutilating the hostages they had given his father for their good behaviour).
In Denmark, he found his brother Harald had succeeded to Svein's throne, and was willing to assist Canute to challenge for England. Ships and men were assembled, and the services of two formidable captains were secured, to bolster the 20-year-old's military skills. Eirik Hladir was the Danish jarl or sub-king of parts of Norway; Thorkell the Tall on the other hand had been Ethelred's man until recently, but now switched to the younger man's service.
Canute landed in England in summer of 1015, and found the kingdom in disarray, not least because of the sinister Eadric Streona who had murdered two noblemen, and encouraged King Ethelred to seize their property and the widow of one. He now elected to switch sides and turned up to offer support to Canute. Wessex and Warwickshire fell to the Danes, and although Ethelred's son, Edmund, tried to link forces with Uhtred of Northumbria, Canute struck north via Nottingham to York, killed Uhtred (giving his earldom to Eirik), and then sailed south for London.
At which point Ethelred elected to give up the mortal coil and be gathered to eternity. Edmund was proclaimed king in his stead and he fought a valiant defensive campaign until Canute offered a compromise whereby Edmund took Wessex (the major sub-division of England) and Canute the rest. [It will surprise no-one to learn that, among the honest brokers of this deal was Eadric Streona, although the exact details of his ulterior motives are lost to history].
With consummate timing, Edmund died only months later, and Canute succeeded to the undivided throne. Thorkell the Tall was rewarded with an earldom in East Anglia, and Eadric received Mercia -- for the few months till Canute executed him. Canute also took time to marry Emma, daughter of the duke of Normandy, thereby both strengthening ties between Engand and the Norman duchy, but also reassuring the Christian population, since she was a very devout woman. [He had previously been sharing a bed with one Aelgifu, a Northampton woman, but she, and her children, were by the marriage excluded from the succession].
He governed firmly, but fairly, taking the last danegeld and using it to pay his fleet off and send it home. In 1019 Harald died and Canute crossed to Denmark, assured himself that his succession there was safe, and left governance there to one of Thorkell's sons, and his own son Hardicanute. Canute also tried to conquer Norway, installing Aelgifu and her son Harald, but their rule there withered, and they returned to Canute's side.
When he died, in 1035, Hardicanute succeeded him in Denmark, whilst in England Harald seized power, first as regent for Hardicanute, then as king in his own (elected) right, holding it long enough for his half-brother to secure Denmark against Magnus of Norway, and assemble a fleet. Harald then died, and Hardicanute took England as well.