Cnut (died 1035), sometimes called Cnut the Great, was king of England (reign 1016 to 1035), Denmark (reign 1018 to 1035), and Norway (reign 1028 to 1035). Following the Danish conquest of England, Cnut became England's king and then succeeded his brother in Denmark two years later. Norway was also under Cnut's control after he ousted Olaf Haraldsson in 1028, an arrangement that lasted until shortly before Cnut's death in 1035.

Cnut travelled extensively between his network of kingdoms and was involved in international affairs to a degree not seen from previous English or Scandinavian rulers. He brought relative stability to England after decades of warfare, but less is known of his tenure as ruler of Denmark and Norway. In spite of ruling three kingdoms, occasionally called the North Sea Empire, Cnut's dynasty was ousted from Norway in 1035 and reached a dead end in England and Denmark in 1042.

Cnut's nickname, the Great, was uncommon in English until recently but is more widely used in Scandinavian sources. In popular culture, he is best known for a legendary story about his attempt to command the tide.

Cnut was the son of the Danish king Swein Forkbeard (reign 986 to 1014). The identity of Cnut's mother is disputed, although she may have been Polish royalty. The 13th-century Knytlinga Saga says Cnut was 37 when he died, corresponding to a birth year of 997 or 998, although Cnut's modern biographers think he was born somewhat earlier, perhaps in the early or mid-990s.

In 1013, Cnut accompanied his father, Swein, on a devastating invasion of England. Viking raids in Britain had been common for decades, including ones that Swein had previously led. After campaigning for several months, Swein was acknowledged as England's king by the end of 1013, and Aethelred the Unready, ruler of England since 978, was forced into exile. Around this time, Cnut married an English noblewoman, Aelfgifu of Northampton, linking him to an important English family as Swein established control.

However, Swein, the first Danish king of England, died in February 1014. It had only been a few weeks since he ousted King Aethelred. Cnut took over his father's Viking army, which proclaimed him king, but they were all caught off guard when Aethelred the Unready returned at the head of a massive army. The Anglo-Saxon Chronicle says that Aethelred marched against Cnut in Lincolnshire and destroyed as much of Cnut's support as he could. Cnut himself escaped back to Denmark, where he plotted his comeback.

Cnut caught a break when Aethelred fell ill the next year and could no longer campaign. Cnut returned to England with a new invading force, and the English leadership fell to Aethelred's son, Edmund Ironside, who initially struggled against Cnut. Edmund had more success after his father's death in 1016, and he tenaciously fought a series of skirmishes and battles against Cnut throughout the summer and fall. The Danish conquest reached its climax at the Battle of Assandun, where Cnut decisively defeated Edmund and killed much of the English nobility. Edmund died a few weeks later, leaving Cnut as England's undisputed king.

After Swein Forkbeard's death in 1014, the Danish throne had passed to Cnut's brother, Harald, while Cnut became England's king in 1016. When Harald died in 1018, Cnut succeeded him, and from then on, he was king of both England and Denmark.

Denmark had been ruled by Cnut's family for generations, but to bolster his standing in England, Cnut married Emma of Normandy, the widow of Aethelred the Unready. Although originally from Normandy, as her epithet would suggest, Emma had been England's queen since 1002 and had gone through the same transition Cnut was experiencing – that of being an outsider adjusting to a position of power in the English court. As Aethelred's widow, she was also a direct link to the previous dynasty. Emma and Cnut married in 1017, allowing Emma to retain her title as England's queen, and soon they had a son, Harthacnut.

However, Cnut does not appear to have repudiated or divorced his first wife, Aelfgifu of Northampton, and may have been a bigamist. Cnut continued to associate with Aelfgifu for nearly two decades, all the way up until his death in 1035. Although a source known as the Thorney Abbey Liber Vitae lists both sides of Cnut's royal family as visitors to the abbey, Cnut's general policy was to keep his two wives – and their respective children – apart. Emma's sphere of influence was in southern England, while her son Harthacnut soon found himself in Denmark. The scant references to Aelfgifu and her sons place them in Scandinavia and northern England.

In the 1020s and 1030s, Cnut set his sights even wider. In 1026, he fought a great battle known as Holy River against a coalition of his rivals from Norway, Sweden, and England. The Battle of Holy River is mentioned in numerous sources, suggesting that it was widely considered a highly significant event, but these sources disagree on almost all of its finer details. There are multiple locations possible for Holy River (although it was most likely somewhere in or near Denmark or Norway), and we have conflicting lists of leaders on each side. Even the winner is unclear, with some sources giving Cnut the victory and others saying he was defeated. It is not even clear whether it was a land battle, a sea battle, or both.

However, the general picture that emerges after Holy River is that Cnut was ascendant in Scandinavia. Within two years, he had ousted Norway's king, Olaf Haraldsson, by flaunting both his wealth and might with a fleet of ships decked out in gold. The victorious Cnut installed his nephew, Haakon Ericsson, as Norway's new regent. After Haakon's death in a shipwreck, Cnut turned to his first wife, Aelfgifu of Northampton, and their son, Swein, to govern Norway.

Remarkably, in 1027, right in between the Battle of Holy River and his takeover of Norway, Cnut managed to travel to Rome. There he met Holy Roman Emperor Conrad II and Pope John XIX. Cnut attended Conrad's imperial coronation and secured a deal with the pope to lessen the tolls for English pilgrims on their way to Rome. Although previous English kings had maintained diplomatic relations with Rome or made pilgrimages there, no English or Danish king had been so directly involved in wider European politics.

Cnut also led a campaign to Scotland in 1027 or 1031 (the year is unclear based on medieval sources) and received submission from three Scottish and Irish kings, while he also claimed to rule part of Sweden. Cnut never controlled Scotland or Sweden directly, but he may have had some degree of influence over them as a powerful neighbor.

By the 1030s, Cnut's rule in England had progressed into one of relative stability, but less is known of his reign in Denmark and Norway. In Denmark, after cycling through various regents, Cnut's young son Harthacnut ruled on his father's behalf. Norway, on the other hand, fell in 1035. Aelfgifu of Northampton and Swein's rule there was remembered as an oppressive one, and Swein died in 1035. Norway reverted to the old dynasty in the form of Olaf's son, Magnus the Good. Aelfgifu and her surviving son, Harold Harefoot, made their way back to England.

In contrast to the constant activity of his early and middle reign, Cnut's travel became more restricted in his final years. Cnut's modern biographers have all speculated that perhaps the king was in worse health during these years, given that this seafaring monarch did not leave southern England after 1033. He died on November 11 or 12 (sources disagree on the date) in 1035. If the late Knytlinga Saga is correct, Cnut was just 37, although most historians have reasoned that Cnut was in his 40s when he died.

Cnut's remaining territory passed to the next generation. His son with Emma, Harthacnut, directly succeeded him in Denmark (reign 1035 to 1042). Meanwhile, Cnut's son with Aelfgifu, Harold Harefoot (reign 1035 to 40), became England's king following a bitter succession dispute. When Harold died in 1040, Harthacnut succeeded him in England (reign 1040 to 1042), briefly reuniting England and Denmark under one king. To express his dissatisfaction with those who had supported his half-brother Harold, Harthacnut had Harold's body exhumed, beheaded, and tossed into the Thames.

Even though the two branches of Cnut's royal family were fierce rivals, Cnut had accomplished something remarkable by ensuring that the succession dispute was solely between his children. Aethelred the Unready's two surviving sons, Edward the Confessor and Alfred, were not serious contenders in the succession crisis. Each of them tried to make good on their claims, but Edward quickly returned to exile after raiding southern England, and Alfred was almost immediately betrayed to Harold Harefoot and killed. The English nobility was wholly devoted to Cnut's dynasty by the time he died in 1035 – the only question was which of his sons they would support.

Cnut had succeeded in passing along two of his kingdoms to his sons, but his direct male line soon reached a dead end. Cnut did have a grandson, Aelfwine, through Harold Harefoot, but Aelfwine entered the church at some unknown point and is not known to have claimed any royal titles.

After Harthacnut's death in 1042, the Danish and English crowns went in different directions. The Danish throne was taken by Magnus the Good (reign 1042 to 47) and then by Cnut's half-nephew, Swein II Estridsson (reign 1047 to 76).

In England, the throne reverted to the Old English royal family in the form of Edward the Confessor (reign 1042 to 1066). Following the Norman conquest of England in 1066, William the Conqueror and his descendants became England's monarchs. Cnut's relatives still occasionally threatened England, though. Swein II Estridsson, for example, attacked England in 1069. Swein II's son, Cnut IV, planned an invasion of England as late as 1084, although it did not come to fruition. Ultimately, the Norman dynasty cemented its control, and the Danish monarchs of England faded into history.

While Cnut's achievements did not outlast him for long, he is the cornerstone of the Danish dynasty in England. Cnut reigned longer than the three other Danish monarchs of England combined. He reclaimed what his father had won in 1013, and he also ensured that the throne passed to his children, Harold and Harthacnut, even though Aethelred the Unready still had living sons and grandsons. Cnut's control of Denmark and Norway also meant that England was part of the Scandinavian world for a time, not merely adjacent to it, which would never happen in this way again.

Even though he was the ruler of three kingdoms, a Viking leader, and a conqueror, Cnut is perhaps best known today for a legend about his attempt to control the tide. The story was recorded by Henry of Huntingdon in the 1100s and has been repeated ever since. In this tale, Cnut brings his fawning advisors to the beach, where he commands the tide to stop. In the original version of the story, Cnut does this to demonstrate that he is not all powerful – only God is. It was a way that Cnut demonstrated his wisdom and taught his overly flattering followers an important lesson.

However, today the story is most often told with a different connotation, in which Cnut genuinely believes he can control the tide. As a result, the story of Cnut and the tide is often invoked to refer to someone who is delusional or believes they can affect reality through their own wishful thinking, especially if they are up against something impossible to control. Originally, though, it reflected a tradition of Cnut as a wise and pious monarch.