Not only did he introduce Christianity throughout what is now Denmark, but he oversaw a touching familial tribute amongst a massive project of construction and fortification.

A royal upbringing

For a man who is credited as an important early member of the Danish royal dynasty (which is still as much a part of Danish society today as it was a millennium ago), the historical record of Harald Gormsson's life is full of gaps and silences. 

His early years are a murky mystery, lost in the midst of time.

What we do know about his formative years is that his father, Gorm the Old, was said to have unified much of what is now Denmark under his rule and founded a royal dynasty that still rules the country today. 

When Gorm died – sometime in the late 950s to early 960s CE - power passed to his son, Harald.

The somber occasion of Gorm's death saw Harald extend an ancient burial mound (believed to have been used since the start of the Nordic Iron Age in approximately 500 BCE) that housed not only his father but also a wealth of grave goods. 

However, this would not be the final resting place for his father.

Conversion to Christianity: by force or persuasion?

Upon ascending to the throne, Harald Bluetooth is said to be singlehandedly responsible for the widespread introduction of Christianity into his Danish kingdom. 

Frustratingly, however, most of what we know was written decades, if not centuries, after the fact. 

A contemporary account from a German chronicler, Widukind of Corvey, claims that he was converted and baptized by a priest named Poppa.

Writing decades later, Adam of Bremen (one of the most famous chroniclers of early medieval Northern European events) recorded that Harald's conversion occurred under duress, forced after a defeat by the Holy Roman Emperor Otto I.

Regardless of the exact details of his conversion – whether it was voluntary or forced - Bluetooth is generally agreed to be one of the main actors in the early promotion, and spread, of Christianity throughout Denmark. 

This was, of course, a "top-down" approach, and the paganism of the Old Norse religion survived well into the late 11th and early 12th centuries.

Harald's new faith, believed to have been adopted sometime in the 960s, saw the removal of his father's body from the burial mound to Jelling. 

Here, not only was a royal palace said to have been constructed (the archeological evidence for this is still being debated), but Harald oversaw the erection of familial tribute to his parents.

The Jelling Stones, a UNESCO World Heritage site, include a stone for King Gorm's wife, Thyra, and a larger one by Harald Bluetooth celebrating his parents and Denmark's Christianization. Photo: Ajepbah / Wikimedia Commons CC BY-SA 3.0

The Jelling Stones and other construction projects

Like so many rulers before and after, Harald had a keen eye for the value of good publicity.

In a touching tribute to his parents, he erected a runestone in their honor (both to his father and mother) while also professing that he had won "all of Denmark" and "turned the Danes to Christianity." 

Neither of these statements is entirely true. 

Harald never ruled all of Denmark, he was particularly strong in the Jutland region but not all of the area that comprises the modern nation-state of Denmark. 

Whilst he may have helped to spread Christianity amongst the Danes, Christian missionaries had been trying to convert the Saxons and the Danes since at least the 6th century CE, if not before.

Aside from his boastful and/or fitting familial tribute, Harald was also responsible for a vast array of construction and infrastructure projects. 

Not only did he refortify and extend the Danevirke – a series of fortifications across the Cimbrian Peninsula – that helped keep invading armies and peoples out of his realm. 

He was also responsible for building the mighty citadel fortress at Aros (modern-day Aarhus) and a series of five ring fortresses constructed in the regions around Aarhus. A sixth, in southern Sweden, may well have been built during his reign too. 

These formed an iron grip on Denmark and, upon their construction, finally ensured that Harald had complete military and economic dominance over the region.

With relative peace and prosperity in his kingdom, he turned his eye towards adventures and intrigues overseas.

Consolidation, conquest, and defeat

A theory put forward in his popular "Hardcore History" podcast by host Dan Carlin was that the Viking explosion and expansion from the mid to late 8th century onwards may well have been a defensive reaction to the campaigns of nearby rulers like Charlemagne. 

The man dubbed "the Great" decimated vast swathes of northern Germany occupied by Danes' next-door neighbors, the Saxons. 

By the time the founder of the Carolingian Empire died in 814, the Frankish realm had extended well into the borders of southern Scandinavia.

With Denmark secure, Harald turned his eye across the Skagerrak. 

Danish people had a presence in southern Norway, especially around the Oslo fjord, since the early 9th century. 

However, following the assassination of Norwegian King Harald Greycloack in 970 (they just don't give royals cool monikers like this anymore), Harald Gormsson was able to subjugate this northern kingdom under his rule. 

Historians are divided on how established his rule was, but he appears to have held some form of power over Norway until his death in 985 or 986.

Ultimately, it was his adventures closer to home that were his undoing. 

His son, Sweyn, by now a young man, had extended his father's rule by sailing across the Baltic and subduing Samland (the region surrounding the Russian enclave of Kaliningrad). 

It was during the 970s that, according to the Norse sagas, Harald was said to have been twice defeated in a naval battle by the Jomsborg Vikings and surrendered one of his daughters, Thyra, as part of a negotiation.

His downfall, grounded in historical records, arose when the Holy Roman Emperor Otto I died in 973. 

Harald launched a swift invasion into southern Saxony at once, demonstrating a historical recurrence of rulers overreaching their ambitions. When the new Emperor, Otto II, counterattacked, Harald lost a part of his southern realm.

Yet worse was to come.

Aerial view of the Trelleborg fortress near Slagelse, Denmark – one of the Viking ring fortresses commissioned by King Harald Bluetooth. Photo: Thue C. Leibrandt / Wikimedia Commons CC BY-SA 3.0

The ultimate betrayal and a legendary nickname

The last years of Harald's life would see a sad demise. His son, Sweyn, groomed for success, led a rebellion against his father that would lead to the father's untimely death. 

Though he was believed to be in his late 60s (a very ripe old age for the early medieval period), he was forced by his son's rebellion to flee into exile, and he died in either 985 or 986. 

Other later medieval sources, such as Adam of Bremen, had Harald dying slightly more dramatically, having led a siege against the Viking stronghold of Jomsborg.

The story of Harald Gormsson, the King of Denmark and Norway, did not conclude with his death. He is now remembered best as "Harald Bluetooth," apparently, due to a bad tooth. 

Centuries later, in 1997, the American technology company Intel invented a new type of wireless technology. A chance conversation between the lead designer and a Swedish colleague saw the adoption of the two Younger Futhark runestones that spelled Harald Bluetooth's initials: H (ᚼ) and B (ᛒ). 

Like Bluetooth, the ruler said to have unified many kingdoms under his rule, this new technology would unify a series of devices. Whilst the nickname may be a dentist's worst nightmare, it has also become part of the fabric of our life in the 21st century.

For more information on a recent archaeological discovery of treasure associated with Harald Bluetooth, visit the BBC here.

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