North Sea Empire

Between 1013 and 1042, near the end of the Viking Age, a short-lived Norse-ruled union called the North Sea Empire—or Anglo-Scandinavian Empire—linked the kingdoms of England, Denmark, and Norway. This was a thalassocracy: a maritime empire whose parts were tied together and sustained only by the sea. The first ruler to hold all three crowns was Sweyn Forkbeard, who had been king of Denmark since 986 and of Norway since 1000, and who conquered England in 1013. He died the next year, and his lands were split apart. His son, Cnut the Great, later took England in 1016, Denmark in 1018, and Norway in 1028. When Cnut ruled all three kingdoms between 1028 and 1035, he was the most powerful monarch in western Europe after the Holy Roman Emperor.

The name "North Sea Empire" was invented by historians in the early 1900s, though the idea that Cnut’s domains formed an empire appeared as early as 1623 in John Speed’s *Histoire of Great Britaine*. At the time, the union was simply called by its separate parts: the kingdoms of Denmark, Norway, and England.

Cnut was the younger son of Sweyn Forkbeard. In early 1014, Sweyn was invading England, and Cnut joined him, commanding the fleet in the River Trent while Sweyn traveled south. Sweyn died on 3 February, and Cnut initially had his Danish troops’ support. But the invasion collapsed when the men of Lindsey failed to supply promised horses. English nobles brought back King Æthelred—whom they had earlier deposed—after he promised reforms, and Cnut returned to Denmark. Cnut’s brother Harald succeeded Sweyn as king of Denmark. With the Danish throne out of reach, Cnut decided to invade England on his own. He raised a fleet by promising shares of plunder and sailed for England in summer 1015, with help from Eric Haakonsson of Norway. England was torn by intrigue among Æthelred, his sons, and other nobles, making the moment right for Cnut. Æthelred died on 23 April 1016 before Cnut could attack in force, sparking a succession crisis. Londoners chose Æthelred’s son Edmund as king, but other nobles swore loyalty to Cnut at Southampton. Cnut blockaded London but was beaten by Edmund at the Battle of Otford. The Danes then moved into Essex and defeated Edmund’s forces at Assandun. Edmund and Cnut agreed that Edmund would keep Wessex while Cnut ruled England north of the Thames. Edmund died on 30 November 1016, leaving Cnut sole king of England. Cnut strengthened his position by marrying Æthelred’s widow Emma in summer 1017 (he was already married to Ælfgifu of Northampton). In 1018, he paid off his invasion fleet—largely with money from London—and was fully accepted as king.

King Harald died childless in 1018 or 1019, leaving Denmark without a ruler. Cnut, as his brother’s heir, went to Denmark in 1019 to claim the throne. While there, he wrote to his English subjects that he was abroad to prevent an unspecified danger, and he returned only to put down rebellions. One Danish chronicle says the Danes had earlier deposed Harald in favor of Cnut, then brought Harald back because of Cnut’s frequent absences. King Olaf II of Norway and King Anund Jacob of Sweden saw the combined Anglo-Danish kingdom as a threat—Cnut’s father Sweyn had dominated both their countries. Olaf attacked Denmark in 1025 or 1026 while Cnut was away. Ulf Jarl, Cnut’s brother-in-law and former Danish regent, and Ulf’s brother joined the fight. Cnut surprised Olaf and defeated the Swedish fleet at the Battle of the Helgeå. Olaf fled, giving up his designs on Denmark. Ulf’s help at Helgeå did not ease Cnut’s suspicions of his treachery. Chronicles say that on Christmas 1026, Cnut ordered a housecarl to murder Ulf in Trinity Church (later Roskilde Cathedral) on Zealand. Cnut went to Rome in 1027—partly as penance for ordering Ulf’s murder, partly to assert himself as an equal to other European monarchs at the coronation of Holy Roman Emperor Conrad II. The trip paid off: Cnut negotiated lower tolls for Northern European pilgrims to Rome and a reduction in the papal tax on the pallium for English archbishops. He persuaded the emperor to return Schleswig and a strip of ancient Danish land between Hedeby and the Eider that the Germans had held. The emperor also agreed to have his son Henry marry Cnut’s daughter Gunnhild.

Olaf II had expanded his power in Norway while Jarl Erik was with Cnut in England. Cnut’s hostility toward him went back further: Æthelred had returned to England on a fleet provided by Olaf. In 1024, Cnut offered to let Olaf rule Norway as his vassal, but after Helgeå he worked to undermine Olaf’s unpopular rule with bribes. In 1028, Cnut set out with 50 ships to conquer Norway. A large group of Danish ships joined him, and Olaf withdrew into the Oslo Fjord while Cnut sailed along the coast, landing at various points and receiving oaths from local chieftains. At Nidaros (now Trondheim), he was acclaimed king at the Eyrathing, and within a few months Olaf fled.