Sweyn had coins made with his likeness, being the first Danish king to do so. The inscription read "Zven, Rex ad Dener" which translates to "Sweyn, king of Danes".Ministry of Foreign Affairs of Denmark (2006). Coinage in Denmark. Official web site. Retrieved 12 Oct. 2006 His father Harald accepted Christianity in the early or mid-960s. The 11th century historian Adam of Bremen reported that when the royal family converted, Sweyn was given the Christian name Otto in honour of the German emperorAdam of Bremen. Gesta II.3. Ed. Schmeidler, trans. Tschan, p. 56. However, Sweyn is not recorded as having used this name officially. He did not use it on his coinage and when he was accepted by the Anglo-Saxon Witan he was referred to as king Sweyn.

Many details about Sweyn’s life are contested. There is an ongoing dispute among scholars over the extent of trust historians may place in the historic sources from the era, such as the Anglo-Saxon Chronicle, Adam of Bremen's Gesta Hammaburgensis Ecclesiae Pontificum and the 13th century Icelandic author Snorri Sturluson’s HeimskringlaHoward, Ian (2003). Swein Forkbeard’s Invasions and the Danish Conquest of England, 991– 1017. Woodbridge: Boydell, 2003. ISBN 0-851-15928-1.. Some historians, including Lauritz Weibull, have argued that Sweyn’s wife, the Swedish dowager queen Sigrid the Haughty, is purely fictional, whereas others have accepted her existence on the evidence of the Norse sagas. In some of the old sources, such as the Jómsvíkinga saga, Sweyn appears as an illegitimate son of Harald, raised by the legendary Jomsviking and jarl of Jomsborg, Palnatoke. Sweyn is depicted as a rebel, leading a 987 uprising against his father, then chasing Harald out of the country, forcing him to flee to Wendland, Germany."Sweyn I" (2006). In Encyclopædia Britannica. Retrieved October 12, 2006, from Encyclopædia Britannica Online. Many negative accounts build on Adam of Bremen's writings; Adam is said to have watched Sweyn and Scandinavia in general with an "unsympathetic and intolerant eye".Sørensen, M. P. (2001). "Religions Old and New". The Oxford Illustrated History of the Vikings. Ed. P. H. Sawyer. Oxford University Press, 2001, p. 202. He accused Sweyn of being a rebellious pagan who expelled German bishops from Scania and Zealand, persecuted Christians, betrayed his father and who was therefore duly punished by his father's pious German friends by being sent into exile, and then deposed in favor of king Eric the Victorious of Sweden, whom Adam wrote ruled Denmark until his death in 994 or 995. However, some modern scholars have found little support for Adam's claim that Sweyn was driven into exile in Scotland for such a long time as 14 years, pointing out that Sweyn built churches in both Lund and Roskilde, as well as led raids against England, during the time he is said to have been deposed and exiled.Lund, Niels (2001). "The Danish Empire and the End of the Viking Age". The Oxford Illustrated History of the Vikings. Ed. P. H. Sawyer. Oxford University Press, 2001, p. 167-181. ISBN 0-19285-434-8. Some scholars have argued that the Anglo-Saxon Chronicle is the work of Archbishop Wufstan II of York and thus created as a propaganda piece against the Danes, in favour of King Edmund Ironside.


Sweyn's grandfather Gorm and father Harold Bluetooth are considered to have laid the foundation for a centralized Danish monarchy. Even so, this foundation appears to have been insufficient in sustaining Sweyn's grip on power in Denmark when he faced a bid by king Eric the Victorious for hegemony over Scandinavia. As a result, Sweyn left to join in the raids against England from 991 to 995, whether deposed or not, fighting as one of a number of warband leaders seeking wealth and tribute. By 994, Swein had been hired as a mercenary by Aethelred II.

After the death of king Eric the Victorious in 995, Sweyn's authority in Denmark grew. The same year, he also started a feud with king Olaf Tryggvason of Norway. With the help of the Swedish king Olaf Skötkonung and Norwegian Eiríkr Hákonarson, Earl of Lade, he defeated Olaf in the Battle of Svolder in 1000.Following the death of king Olaf in the battle, Sweyn shared sovereignty with his allies, establishing Danish control over most of Norway by posing Eiríkr Hákonarson as his vassal. Most historical sources agree that Sweyn used both Danish and Norwegian forces in a combined Viking assault on England.