Swein Forkbeard (died 3 February 1014) was King of Denmark from and briefly King of England from December 1013. He was also overlord of Norway and from c. 1000. He married the sister of Boleslaus, ruler of Poland, and had three known children by her: King Harald II of Denmark, King Cnut of England, and Estrid, the mother of Sweyn Estridsson, who became king of Denmark in 1047. Swein also had a daughter by an earlier liaison, Gytha, whose son Hakon Eriksson was born in 998.

Around 986 Swein rebelled against his father, King Harald Bluetooth, and seized the throne of Denmark. Harald was driven into exile and died shortly afterwards. In 992 or 993 Swein defeated an invasion by Erik, the king of the Svear (a people in eastern Sweden). Swein probably became overlord of the Svear and Erik died in the war or soon afterwards. In 994 Swein raided England together with the Norwegian warlord Olaf Tryggvason, until they were paid off by a tribute of 16,000 pounds. Swein had inherited the overlordship of Norway from his father, but Olaf challenged Swein's control and was recognised by many Norwegian magnates as king. In 999 or 1000, Swein defeated Olaf at the naval Battle of Svold; Olaf was killed in the battle and Swein recovered control of Norway. Swein returned to England in 1003 and ravaged until 1005, when a major famine forced him to return to Denmark.

Swein is not known to have returned to England until 1013, when he invaded intent on conquest and seizure of the English throne. By December he had achieved full control and was acknowledged as king. After spending Christmas on the Isle of Wight, Æthelred the Unready, the Anglo-Saxon king of England, fled to his brother-in-law, Richard, Duke of Normandy. Swein's reign was very brief as he died on 3 February 1014.

Harald Bluetooth had converted to Christianity, and Swein carried on his pro-Christian policies, although historians disagree whether his support was motivated by religious devotion or political calculation. He is praised by historians as a great Viking warrior and an able king.

Sources Little is definitely known about Swein's life. Most accounts have been based on Adam of Bremen's Deeds of the Bishops of Hamburg, which dates to the 1070s. Adam was a strong supporter of the claims of the German emperor and archbishop of Hamburg–Bremen to have authority over Denmark. The historian Peter Sawyer states that Adam was hostile to Swein and there is much which is clearly false in his account: "Adam denigrated Swein because the latter neither recognized the German emperor as his overlord nor acknowledged the ecclesiastical authority of the archbishop of Hamburg–Bremen, which in Adam's eyes was tantamount to paganism." Other scholars, such as the Welsh scholar Gwyn Jones and the Danish historian Niels Lund, also regard Adam as very unreliable. Adam claimed that Swein apostasised and persecuted the church, and as punishment by God he was defeated by King Erik of Sweden and spent fourteen years in exile in Scotland. Lund comments that other sources give a very different account, and "they are so much more reliable that we have little reason to doubt that Adam invented his story". The chronicler Thietmar of Merseburg, who was a contemporary of Swein, was also hostile to him.

The Encomium Emmae Reginae, commissioned by Swein's daughter-in-law Emma in the early 1040s, is equally biased in the opposite direction, praising Swein. Other sources include the Anglo-Saxon Chronicle and skaldic verses in later Icelandic sagas, such as a verse which refers to Swein "bloodying swords in England with God's help". Swein's byname, "Forkbeard", is first recorded around 1140 in the Roskilde Chronicle.

Background In the ninth century, Anglo-Saxon England came under increasing attack from Viking raids, culminating in an invasion by the Great Heathen Army in 865. By 878, the Vikings had overrun the kingdoms of Northumbria, East Anglia, and Mercia, and nearly conquered Wessex, but in that year the West Saxons achieved a decisive victory at the Battle of Edington under King Alfred the Great (). Over the next fifty years, the West Saxons and Mercians gradually conquered the Viking-ruled areas, and in 927 Alfred's grandson Æthelstan () became the first king of all England when he conquered Northumbria. He was succeeded by his half-brothers Edmund () and Eadred (), who both lost control of York (southern Northumbria) at the start of their reigns, but had recovered it by the time of their deaths. There was then a generation of peace, until Viking attacks resumed with small-scale raids from Scandinavia in the early 980s.

Denmark's power declined in the late ninth and early tenth centuries, and its revival probably began under Swein's grandfather, Gorm the Old, who died around 958. He was succeeded by his son Harald Bluetooth, who extended Danish royal authority and claimed on the inscription on one of the Jelling stones that he had "won the whole of Denmark for himself, and Norway, and made the Danes Christian." He controlled the area around the Oslofjord in Norway and was acknowledged as overlord by the most powerful Norwegian ruler, Haakon Sigurdsson, Earl of Hlathir. Harald paid homage to the German emperor, Otto I, but when Otto died in 973 Harald unsuccessfully attempted to defy his son and successor, Otto II. In 982 Otto was defeated by the Muslim ruler of Sicily at the Battle of Stilo in Calabria, and the following year Harald campaigned successfully against the Germans, thus removing any imminent threat of German invasion. Gorm had been a pagan, but Harald converted to Christianity in the mid-960s.

Life Early life Swein was a son of Harald Bluetooth and an unknown mother. It is not known when Swein was born, but his grandson Hakon Eriksson was born in 998.

King of Denmark and Viking raider In 986 or 987 Swein revolted against his father with the support of sections of the nobility and seized the Danish throne. Harald was defeated and wounded in battle against his son. He was driven into exile, where he died of his wounds shortly afterwards. In the view of the historian Timothy Bolton, the family turned in on itself, "unleashing a brief civil war that ended in a scandalous regicide and apparently leaving a deep scar on Danish memory". Jones describes Harald's expulsion as an "astonishing reversal", probably due to erosion of the nobility's ancient rights and hostility to Christianity, particularly because the church was pro-German.

Swein was able to extend Danish control east of Øresund (a strait which now forms the border between Denmark and Sweden) into Skane in what is now southern Sweden. The main centres of Danish power shifted from Jutland to the towns of Roskilde and Lund on each side of the strait. Harald and Swein unified Denmark in the second half of the tenth century, and some aristocrats lost out and joined bands of Viking raiders on England. Swein was secure enough at home to become the first Scandinavian king known to have led Viking raids. His armies comprised his own retainers and the bands of other Viking warlords, drawn from all over Scandinavia. The main Viking target was England, which under King Æthelred the Unready was both wealthy and militarily weak, and Swein's ability to extract large amounts of tribute enhanced and extended his power in Scandinavia. The Encomium states that Swein made his men "submissive and faithful to himself by manifold and generous munificence", and the Danish historian Niels Lund comments that "the world in which Swein was operating was the traditional early medieval world in which princes bought loyalty by means of gifts and political relations were maintained by a subtle blend of coercion and reward".

In 991 the English suffered a devastating defeat against a Viking army at the Battle of Maldon in Essex, and they paid 10,000 pounds in tribute to get the invaders to leave. The Anglo-Saxon Chronicle names the Viking leader as the Norwegian warlord Olaf Tryggvason, but historians think that the Viking army was probably also led by Swein. The confirmation by King Æthelred of the will of Æthelric of Bocking between 995 and 999 refers to an allegation many years before Æthelric died that he had been involved in a plot to receive Swein in Essex when he first came to England, and this is thought to refer to the raid in 991; Sawyer suggests that Swein was the senior commander over Olaf.

The Svear in eastern Sweden resented Danish control of the trade route to the North Sea, and in 992 or 993 Erik, King of the Svear, attacked Denmark. Swein defeated the invasion and Erik died either during the war or soon afterwards. Erik was succeeded by his son Olof, whose byname "Skötkonung" means tributary king. Lund and Sawyer see Olof's byname as evidence that he acknowledged Swein as his overlord, but in the view of the Norwegian historian Sverre Bagge the relationship between Olof and Swein is not known.

Swein and Olaf Tryggvason invaded England in 994. They unsuccessfully attacked London, and then raided around the south-east until the English agreed to pay a tribute of 16,000 pounds. Swein then probably sailed west as, according to the Annales Cambriae, "Swein son of Harald" ravaged the Isle of Man. King Æthelred persuaded Olaf to enter his service, and then encouraged him to return to Norway to challenge Swein's overlordship. Olaf was recognised as king by many Norwegian magnates, and Æthelred was thus successful in distracting Swein from attacking England for several years. Olaf was killed in 999 or 1000 in the naval Battle of Svold against Swein, who was supported by Olof Skötkonung and Hakon's sons. Swein recovered his overlordship over Norway, and he appointed Hakon's sons, Erik Hakonsson (his son-in-law) and Swein Hakonarson to rule as jarls (earls).

Southern coastal areas of England suffered from Viking raids in 997 to 999, and Swein returned in 1003. He sacked Exeter, defeated local armies of Wiltshire and Hampshire, ravaged Wilton and then returned to his ships. The next year Swein attacked East Anglia, sacking Thetford and Norwich, and then met an army led by Ulfcytel. The Vikings were again victorious, but only at the expense of heavy losses, and then escaped back to their ships. In 1002 King Æthelred had ordered the St Brice's Day massacre of Danish people in England, and post-Conquest chroniclers attributed Swein's return to a desire to avenge the massacre. The twelfth-century historian William of Malmesbury stated that Swein's sister Gunhilde was a victim of the massacre along with her husband Pallig. Twentieth-century historians such as Frank Stenton and Gwyn Jones thought that Swein returned to avenge his sister's death, but twenty-first century historians are sceptical because William is the only source for Gunhilde's existence and Pallig deserted the English before the massacre. Ann Williams and Levi Roach think that the story is a myth, and Sawyer ignores Gunhilde and the massacre in his biographical article about Swein. Bolton suggests that Swein led the raids on England in order to avert the danger of his subordinates making so much money that they were in a position to threaten his position in Denmark.

A severe famine forced Swein to return to Denmark in 1005. The Viking army returned in 1006, and ravaged at will, defeating every English army sent against it until bought off with a tribute of 36,000 pounds. Swein is not named by the Anglo-Saxon Chronicle among the Viking leaders between 1006 and 1013. Some leaders may have been his followers, but Thorkell the Tall, who ravaged England between 1009 and 1012, was an independent warlord. After extracting tribute of 48,000 pounds he went over to Æthelred with forty-five ships, and served him faithfully until the king's death in 1016.