Sweyn was the son of Harald I Bluetooth and was born in April 963. It is believed that Sweyn’s mother was Harald’s first wife Gunhild. Sweyn became king of Denmark around 986 after rebelling against his father. It is possible that Harald was forced into exile and then died shortly afterward in 986 or 987. Sweyn remained king until his death in 1014.
Sweyn is probably best known for his expeditions against Norway and England. Harald Bluetooth had controlled parts of Norway, but his foothold had been lost in the 970s. Olaf I Tryggvason was king of Norway at this point, and Sweyn formed an alliance with the Swedish king Olof Skötkonung to pursue their mutual interests in acquiring parts of Norway. Sweyn and Olof and their allies attacked Olaf’s fleet as he was traveling to (or from, it’s not clear) Pomerania near the island of Svolder. Olaf’s fleet was badly outnumbered and all of his ships were captured or destroyed and Olaf himself jumped overboard rather than be captured by his enemies. The location of “Svolder” has been lost to time, but it’s believed to have been in the western Baltic Sea.
The victorious allies partitioned Norway between themselves. The area along the southern coast of Norway came under direct Danish control, but the rest of Sweyn’s share was governed by the Jarl of Lade as a fiefdom from Sweyn.
Sweyn’s expeditions against England are probably better known. Without getting bogged down with a lot of details, Sweyn had led, or organized, several raiding expeditions against England, beginning in 1002. Supposedly the first raid was motivated by the St. Brice’s Day massacre of Danes ordered by the English King Æthelred in 1002. Sweyn’s sister and brother-in-law were among the victims of the massacre.
In 1013 Sweyn led a much larger force and invaded England with the goal of deposing Æthelred and taking control of the kingdom. The only significant resistance that the Danish forces encountered was in London, but as the surrounding areas submitted to Sweyn, eventually London also surrendered. Æthelred and his sons went into exile and on Christmas Day 1013 Sweyn became King of England.
Sadly for Sweyn, his reign over England lasted only 5 weeks. He died on February 13, 1014 at his base in Gainsborough, Lincolnshire.
Sweyn had eight children by his two wives. Harald II Svendsen and Canute the Great both became kings of Denmark. His daughter Estrid Svendsdatter was the mother of King Sweyn II Estridsson.