The northwest region of the Frankish realms, bordered by the English Channel on the north and the Seine Valley in the south, soon began to buckle under Viking attacks from the mid-9th century CE.

Following the riches gained after the sack of Paris, coastal and riverside communities soon began to spot the ominous sight of more longboats. 

By 851 CE, Vikings began to overwinter throughout what is now northwestern France.

Historians have long speculated about why the Vikings decided to stay longer from this period onward.

One theory is that the bloody wars of Charlemagne saw war and insecurity come to the very borders of the Viking world. These Viking attacks were, as the theory goes, part of a defensive push to secure their southern flank and make sure no Frankish armies would march over the Danevirke.

Another theory is that unlike much of Scandinavia, the Frankish realms saw rapid re-urbanization in the early medieval period with the boom of many market towns. 

These were easy targets for Vikings, who had one of the greatest pieces of early medieval naval technology in their arsenal, able to launch surprise attacks across vast oceans, coastal communities, and as far inland as the Loire Valley.

Finally, remember that Charlemagne had tried to convert many of these "north men" by the point of the sword. 

He had established a glorious palace at Aachen, and the lure of riches and glory was too tempting for many a Viking.

Between 850 and 870, the Vikings conducted raids reaching as far inland as Clermont and as far south as Avignon. They devastated the Aquitaine region and sacked Chartres twice, in 858 and 865.

Yet worse was to come before the end of the century. The Vikings would strike at the very heart of the Frankish empire.

The beginnings of settlement

Towards the later stages of the 9th century, the Frankish realms were under constant attack by Vikings.

The Vikings, it appeared to the Carolingian kings, were everywhere. 

When they were not raiding the northern areas of the Frankish realms, modern-day Netherlands, Belgium, and northeast France, they were striking in the Mediterranean to decimate coastal communities along the southern coast of France.

However, this increased raiding would eventually lead to one thing - settlement.

Over the course of a century, following events in Anglo-Saxon England across the Channel, what started as minor predatory raids became an existential threat to the Frankish rulers.

Charlemagne was christened the first Holy Roman Emperor in 800, but his Carolingian successors failed to live up to his greatness. 

Upon the death of a ruler, the political pendulum would swing wildly, resulting in a lack of unity in purpose. When one Emperor passed away, his successor often acted in direct contradiction to his predecessor's actions.

In 867, the Treaty of Compiegne serves as a prime example. 

Charles the Bald ceded part of the Cotentin peninsula to a Breton king in exchange for a pledge of loyalty and assistance in repelling Viking raiders. 

This marked the first instance of a Frankish ruler giving up territory to counter a Viking threat, but it wouldn't be the last.

In the region now known as Normandy (see below), Vikings had not only raided and decimated coastal communities for generations by the turn of the 10th century but were also starting to replace the local Frankish population.

Raiding and trading were fine for a few generations, but the Viking homeland (in the Scandinavian and Baltic regions) appears to have had a population push and plentiful settlers willing to cross the seas for a new life.

Though these people plucked from Viking societies were vastly different from the local Frankish populations in language, customs, religion, and appearance – they soon began to mix, mingle, and intermarry. 

However, this was not the case all over the Frankish realms, as we shall see in Chartres.

Rouen, captured by Rollo in 876, became a strategic Viking stronghold in central France. Photo: Sergii Figurnyi / Shutterstock

The siege of Chartres and the birth of the Normans?

Perhaps the most consequential of these Viking raids was the siege of Chartres in 911. 

Whilst the town had been sacked and raided twice in the mid-9th century, the 911 siege was led by a ferocious Viking warrior called Rollo

In the odd half-century since the last Viking attack, the town had improved its fortifications and defences, but, unfortunately, Rollo was renowned as one of the most brilliant Viking commanders of the early medieval period.

Having established a foothold in the region by capturing Rouen in 876, Rollo had a base to strike fear into central France. 

The siege of Chartres saw the town surrounded, and a Frankish force rode to the rescue led by Richard, Duke of Burgundy, and Robert I of Neustria. 

Robert had succeeded the last of the Carolingian kings in a coup and has been called the first king of France by some historians.

Nevertheless, if we are to believe contemporary Frankish chroniclers, a holy relic (a tunic of Christ) was said to have scared the Vikings away from Chartres, with Rollo somehow managing to escape. 

However, Robert knew he had won the battle but not the war as Rollo and his small band of men laid waste to the countryside around Rouen. 

Rollo oversaw a scorched earth policy, leading Robert to the negotiating table.

The Treaty of Saint-Clair-Sur-Epte, signed in 911, was the most remarkable political achievement of the Vikings in the 10th century.

Here, Rollo met Charles the Simple in person to hash out a peace agreement. 

Charles, either justifying his epithet or contradicting it (depending on which modern historian you read), ceded a vast swath of territory between the Loire and Seine rivers to Rollo.

In return, Rollo would further settle the area with his Viking brethren and serve as a bulwark against additional Viking attacks.

This area would soon be named after the Latin term for these Viking "northern men" - Normanni. Or, in the Frankish tongue, it became "Normandy."

From the shores of Scandinavia to the heart of Normandy, the Vikings' journey culminated in a melding of cultures that shaped the course of European history. Photo: canadastock / Shutterstock

Normandy and later events

Whilst Viking raids would continue to haunt France for a century and a half after the establishment of Normandy, it is here we must end our history. 

The establishment of Normandy saw the Viking settlers combine with the local Frankish population to become, in a generation or two, Normans. 

The Normans mixed Viking expansionism with ruthless Frankish martial skills. 

Their success and exploits, throughout Europe, between the 11th and 13th centuries mirror the meteoric rise of their Viking ancestors centuries before. 

Their influence on Normandy today can be seen in the region's language, governance, and strong culture.

A generation or two after the establishment of Normandy, the Carolingian rulers fell from power in an oh-so-Frankish power struggle. 

In 987, Hugh Capet, the count of Paris, led a dynastic coup that saw his accession to the throne. 

Though he would rule only a small area between the middle and lower reaches of the Seine, his dynasty would expand over the next three and a half centuries to include much of what is now France. 

The modern French Republic can trace its ancestral roots back to the ascension of Hugh Capet as king of the Franks over a millennium ago.

As the Viking history of France ended with the establishment of Normandy and Hugh Capet's ascension to power, the medieval history of France had just begun.

For more on the history of the Normans, visit the BBC History Extra website here.

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