King Sweyn I of Denmark, often remembered by his nickname 'Forkbeard', was the first Dane to rule the kingdom of England. His reign was very short, and his most significant role in the history of England is that of a raider and warlord who conducted terrible Viking raids, extracting huge payments from King Ethelred the Unready in exchange for temporary truces.
The son of Harald Bluetooth, King of Denmark, Sweyn began his reign after leading a rebellion against his father, during which the old king perished. Historic sources disagree as to the details of his life during the next few years, but he may have been driven out of Denmark and into exile in Britain. His first recorded expedition in England was in 993, when he and the Norwegian Olaf Tryggvason besieged London with a fleet of 94 warships. The siege was unsuccessful, but the Vikings plundered the surrounding countryside and coastal areas so extensively that King Ethelred was forced to come to terms with them, eventually paying them off with £16000. Whereas Olaf accepted Christianity and promised never to attack England again, Sweyn returned to Scandinavia without making any such promise, and instead turned against his old ally, eventually defeating Olaf and adding Norway to his territories around the turn of the millennium. Danish raids in England continued with little respite throughout the 990s.
There were violent reprisals in 1002, when the so-called St Brice's Day massacre of Danes who had settled in England took place on the orders of King Ethelred, who feared assassination at their hands. According to some accounts, Sweyn's own sister died in the massacre, and Sweyn thereafter pursued a vendetta against Ethelred, with devastating raids in the southern half of England that worsened as disillusioned Anglo-Saxon noblemen began to collude with the Danes. Finally, in 1013, Sweyn (accompanied by his son Cnut) began to receive the submission of English earls, and Ethelred fled into exile in Normandy. Sweyn was acclaimed king by an exhausted and terrorised populace, but his reign lasted for a matter of weeks; he died suddenly at Gainsborough in 1014. One account describes him falling from his horse, but a later legend has him slain in his sleep by the English St Edmund, who had himself been martyred by Vikings in the ninth century. The ousted King Ethelred returned to the throne until his death in 1016.
Although it is difficult to see Sweyn's impact on England in anything other than a negative light, he prepared the way for his son, Cnut, to eventually succeed him in England, and to rule over an empire that also included Norway and Denmark.