information:extraction, linguistic:translation
Birth and kingship

Cnut was a son of the Danish prince Sweyn Forkbeard, who was the son and heir to King Harald Bluetooth and thus came from a line of Scandinavian rulers central to the unification of Denmark. Neither the place nor the date of his birth are known. Harthacnut I was the semi-legendary founder of the Danish royal house at the beginning of the 10th century, and his son, Gorm the Old, became the first in the official line (the "Old" in his name indicates this). Harald Bluetooth, Gorm's son and Cnut's grandfather, was the Danish king at the time of the Christianization of Denmark; he became one of the first Scandinavian kings to accept Christianity.

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, 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). Adam of Bremen in Gesta Hammaburgensis ecclesiae pontificum is unique in equating Cnut's mother (for whom he also produces no name) with the former queen of Sweden, wife of Eric the Victorious and by this marriage mother of Olof Skötkonung.

To complicate the matter, Heimskringla and other sagas also have Sweyn marrying Eric's widow, but she is distinctly another person in these texts, named Sigrid the Haughty, whom Sweyn only marries after Gunhild, the Slavic princess who bore Cnut, has died. Different theories regarding the number and ancestry of Sweyn's wives (or wife) have been advanced (see Sigrid the Haughty and Gunhild). But since Adam is the only source to equate the identity of Cnut's and Olof Skötkonung's mother, this is often seen as an error on Adam's part, and it is often assumed that Sweyn had two wives, the first being Cnut's mother, and the second being the former Queen of Sweden. Cnut's brother Harald was the younger of the two brothers according to Encomium Emmae.

Some hint of Cnut's childhood can be found in the Flateyjarbók, a 13th-century Icelandic source that says he was taught his soldiery by the chieftain Thorkell the Tall, brother to Sigurd, Jarl of Jomsborg, and the legendary Jomsvikings, at their stronghold on the island of Wollin, off the coast of Pomerania. His date of birth, like his mother's name, is unknown. Contemporary works such as the Chronicon and the Encomium Emmae, do not mention this. Even so, in a Knútsdrápa by the skald Óttarr svarti, there is a statement that Cnut was "of no great age" when he first went to war. It also mentions a battle identifiable with Sweyn Forkbeard's invasion of England and attack on the city of Norwich, in 1003–04, after the St. Brice's Day massacre of Danes by the English, in 1002. If Cnut indeed accompanied this expedition, his birthdate may be near 990, or even 980. If not, and if the skald's poetic verse references another assault, such as Sweyn's conquest of England in 1013–14, it may even suggest a birth date nearer 1000. There is a passage of the Encomiast (as the author of the Encomium Emmae is known) with a reference to the force Cnut led in his English conquest of 1015–16. Here (see below) it says all the Vikings were of "mature age" under Cnut "the king".

A description of Cnut appears in the 13th century Icelandic Knýtlinga saga:

Knut was exceptionally tall and strong, and the handsomest of men, all except for his nose, that was thin, high-set, and rather hooked. He had a fair complexion and a fine, thick head of hair. His eyes were better than those of other men, being both more handsome and keener-sighted.

Hardly anything is known for sure of Cnut's life until the year he was part of a Scandinavian force under his father, King Sweyn, in his invasion of England in summer 1013. Cnut was likely part of his father's 1003 and 1004 campaigns in England, although the evidence is not firm. The 1013 invasion was the climax to a succession of Viking raids spread over a number of decades. Following their landing in the Humber, the kingdom fell to the Vikings quickly, and near the end of the year King Æthelred fled to Normandy, leaving Sweyn Forkbeard in possession of England. In the winter, Sweyn was in the process of consolidating his kingship, with Cnut left in charge of the fleet and the base of the army at Gainsborough in Lincolnshire.

On the death of Sweyn Forkbeard after a few months as king, on Candlemas (Sunday 3 February 1014), Harald succeeded him as King of Denmark, while the Vikings and the people of the Danelaw immediately elected Cnut as king in England. However, the English nobility took a different view, and the Witenagemot recalled Æthelred from Normandy. The restored king swiftly led an army against Cnut, who fled with his army to Denmark, along the way mutilating the hostages they had taken and abandoning them on the beach at Sandwich in Kent. Cnut went to Harald and supposedly made the suggestion they might have a joint kingship, although this found no favour with his brother. Harald is thought to have offered Cnut command of his forces for another invasion of England, on the condition he did not continue to press his claim. In any case, Cnut succeeded in assembling a large fleet with which to launch another invasion.