The North Sea Empire, stretching across the North Sea to encompass the medieval kingdoms of Denmark, England, and Norway, has been seen as a high-water mark for Viking power and influence. 

While the history of the Vikings began with predatory raids from the mid-8th century, it was this Empire (lasting only a generation, from 1013 to 1042) that saw the personal union of three crowns under Sweyn Forkbeard and his son, Cnut the Great

Forkbeard, perhaps the most brilliant Viking commander and politician of the age, was the first to unite these three kingdoms under personal rule. 

According to modern historians, he was the most powerful ruler in Western Europe, second only to the Holy Roman Emperor. Not bad for someone whom contemporary Frankish and Byzantine courts dismissed as a "mere barbarian." 

From raiding small coastal villages, the Vikings established an empire that stretched across two seas and three kingdoms and encompassed a vast number of people, cultures, and languages. 

Yet, as soon as Forkbeard had forged the empire, he dropped dead in 1014, having personally led a successful invasion of England. 

Suddenly, the Empire splintered as his eldest son, Harald, ascended to the throne of Denmark, while a second son, Cnut, was proclaimed King of England by people in the Danelaw

However, the return of the Anglo-Saxon King Aethelred (the actual king and not the Vikings character) from exile saw the throne slip from Cnut's grasp. 

This was said to be done with the help of Olaf Haraldsson, who would later declare himself King of Norway starting in 1016.

Just two years after his father's death, what Cnut no doubt saw as his birthright had slipped from his grasp, with a combination of rivals and family members taking power. 

Though Cnut was down, one does not end up with the epithet "the Great" by accepting a sudden downturn in fortune. He would rise again. 

 In 1017, Cnut's coronation took place at the original site of Canterbury Cathedral, a structure that would later undergo a complete rebuild from 1070 to 1077. Photo: Alexey Fedorenko / Shutterstock

Seizing crowns back one at a time 

Cnut retreated to his homeland, where he bided his time. He was preparing to sail. 

In 1015, with an army of, according to later medieval chroniclers, some 10,000 men in over 200 longships, he set out to seize back the throne of England from Aethelred. 

The invasion proved successful; he dispatched the forces of Aethelred, laid siege to London, and was crowned King of England in Canterbury in 1017. 

One crown down, two to go for Cnut. 

His older brother, Harald II, died a year later, and Cnut rushed back to affirm his succession to the Danish throne. 

He fended off some opposition in Pomerania (a region of northern Poland that was part of the Viking cultural sphere during the early medieval period), but there was more brewing across the border in Sweden. 

With the death of Swedish King Olof Skotkonung in 1022, his son, Anund Jacob, decided to ally with the Norwegian King, Olaf II. 

This posed a strategic problem for Cnut, who aimed to dominate much of Western Europe and simply could not tolerate a strategic weakness in what he saw as his backyard, the Baltic Sea.

Sailing to England in the mid-1020s to reassert his dominance there, Cnut was left exposed as Olaf II, and Anund began to launch attacks on his forces, possessions, and trade in the Baltic Sea. 

However, Cnut gathered an army and sailed back to confront his foes head-on in 1028. 

Though the exact size of Cnut the Great's flotilla at the Battle of Helgeå is unknown, it's estimated to have been around 600 ships, including a notably large dragon ship over 80 meters long. Photo: aerophoto / Shutterstock

Unholy war at a holy river 

According to the sagas and historical sources, Cnut sailed from England with a large force of men and ships. 

Though we may never know the actual size of his flotilla, contemporary estimates put the number at around 600 ships, including a massive dragon ship, over 80 meters (260 feet) long, that Cnut himself was said to sail in. 

As a would-be king of three kingdoms, one would expect nothing less. 

Though smaller in number, the joint Norwegian-Swedish forces waited to launch a surprise attack on Cnut's contingent. 

Lying upstream at the estuary of the river Helge (believed to be somewhere in the modern Swedish provinces of Skane or Uppland), a small force of Cnut's ships sailed nearby. 

At this moment, the Norwegian-Swedish forces were said to release a temporary dam, flooding the river, sinking a multitude of ships, and causing the deaths of untold numbers. 

Yet, this was not the end for Cnut. He had only sent a small expeditionary force near the estuary so he could simply sit back and wait. 

The sources are divided over what happened next. 

According to the Anglo-Saxon Chronicle, this sneaky maneuver won the day for the Norwegian-Swedish force. 

It further mentions how their men won the day on the battlefield, implying a land-based battle along with the naval engagement. 

However, writing centuries later, Snorri Sturluson, in the Saga of Olaf the Holy, claims that though the dam-busting damaged some of Cnut's massive flotilla, the majority of his naval forces lay in wait. 

It was Cnut who would win the battle – probably due to sheer strength of numbers – and would emerge as the de facto ruler of the entire Viking world, or at least, the British Isles and the Viking homeland of Scandinavia. 

Regardless of what exactly transpired, Cnut emerged from this battle as strategically dominant. 

He then set his sights on the final piece of his father's hard-won empire: Norway. Cnut set off with a fleet of only 50 ships to try and seize the Norwegian throne. 

Having done his homework, by bribing much of the Norwegian nobility and political elite, he dispatched Olaf's forces and was crowned King of Norway in Trondheim shortly after. 

He now controlled swathes of eastern Sweden, extending the North Sea Empire into a fourth country. 

Marked in red, this map displays the territories under Cnut the Great's control around 1028, reflecting his successful expansion and consolidation of power. Source: victor falk / Wikimedia Commons (CC BY-SA 3.0)

The dream of empire vanquished forever 

Exceptionally frustrating for modern historians and history lovers, we know so little about the events of the Battle of Helgeå. 

Though it was said to be a great naval battle with, if we believe the sagas, as many as 1,000 ships taking part, we are left in historical darkness. 

What can be assumed is that whatever happened at the estuary of the river Helge saw Cnut the Great emerge as the dominant political player and go on to seize the Norwegian throne, thus winning back the Empire that his father had so bloodily won. 

He would go on to reign for another seven years until his death in 1035. 

In a case of history repeating itself, the empire that a father had forged would splinter after his death, leaving his son, Harthacnut, devoid of his birthright. 

He would go on to rule in both Denmark and England but could not win back the throne of Norway. 

He died in 1042, along with the short-lived dream that was the North Sea Empire.

The Form, a weekly BBC arts & culture podcast, has a whole episode dedicated to Cnut the Great, available to listen to here

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