Historiographical sources on Sweyn's life include the Anglo-Saxon Chronicle (where his name is rendered as Swegen),[7] Adam of Bremen's 11th-century Deeds of the Bishops of Hamburg, and Snorri Sturluson's 13th-century Heimskringla.

[9] Sweyn married the widow of Erik, king of Sweden, named "Gunhild" in some sources,[10] or identified as an unnamed sister of Boleslaus, ruler of Poland.

[4] Historian Ian Howard describes Sweyn as "a competent military commander, politician and diplomat" who made "a formidable and successful king.

[4] Adam of Bremen depicted Sweyn as a rebellious pagan who persecuted Christians, betrayed his father and expelled German bishops from Scania and Zealand.

[12] According to Adam, Sweyn was punished by God for leading the uprising which led to king Harald's death, and had to spend fourteen years abroad (i.e. 986–1000).

[13] The allies attacked and defeated king Olaf in the western Baltic Sea when he was sailing home from an expedition, in the Battle of Svolder, fought in September 999 or 1000.

In part, this reflected the fact that there were numerous Christian priests of Danish origin in the Danelaw, while Sweyn had few personal connections to Germany.

Sweyn's preference for the English church may also have had a political motive, because German bishops were an integral part of the state.

Simon Keynes regards it as uncertain whether Sweyn supported these invasions, but "whatever the case, he was quick to exploit the disruption caused by the activities of Thorkell's army".

King Æthelred sent his sons Edward and Alfred to Normandy, and himself spent Christmas on the Isle of Wight, and then followed them into exile.

[20] Based in Gainsborough, Lincolnshire, Sweyn began to organise his vast new kingdom, but he died there on 3 February 1014,[21] having ruled England for only five weeks.

However, the English nobility sent for Æthelred, who upon his return from exile in Normandy in early 1014 managed to drive Cnut out of England.

After Harthacnut's death, the English throne reverted to the House of Wessex under Æthelred's younger son Edward the Confessor (reigned 1042–1066).

The Chronicon of Thietmar of Merseburg and the Encomium Emmae report Cnut's mother as having been Świętosława, a daughter of Mieszko I of Poland.

Norse sources of the High Middle Ages, most prominently Heimskringla by Snorri Sturluson, also give a Polish princess as Cnut's mother, whom they call Gunhild and a daughter of Burislav, the king of Vindland.

Since in the Norse sagas the king of Vindland is always Burislav, this is reconcilable with the assumption that her father was Mieszko (not his son Bolesław).