“The reverberations lasted for generations” | Matthew Gabriele and David M Perry on the civil war of Charlemagne's grandsons

The three brothers and their nephew basically argued about the form that the empire should take. Should it be coherent and unified under Lothar, or would Charles and Louis be able to function as independent kings in their own domains?

This dispute boiled over into a massive, bloody battle fought on 25 June 841. For most medieval battles, we have just a sense of where and when they were fought. For this clash, though, we have very specific details of the day and the time when the battle started, as well as the location. That’s because – as we argue at the start of our new book – it was so horrifying for everybody involved. It shook the empire and its participants to their core, creating reverberations that lasted for generations.

What’s the backstory to the dispute?

Matthew Gabriele: The Franks rose to power in the fifth and sixth centuries when, after interacting with the Romans for a long time, they were finally able to establish their own realm. The resulting empire was ruled for several centuries by a dynasty called the Merovingians. They were supplanted in 751 in a coup d’état by high nobles who started a new dynasty that, over time, came to be named for one of their progenitors, Charles Martel – Charles being Carolus in Latin, hence the dynastic name ‘Carolingian’.

The apogee of that empire came under Charlemagne, who ruled for a substantial length of time, taking power in 768 and dying in 814. He expanded the Frankish empire to encompass most of western and central Europe, from Denmark in the north down to central Italy, across the Pyrenees into Catalonia and east into Saxony.

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But what’s really interesting to us is how quickly it fell apart – within just two generations. This myth of unity that the Franks constructed for themselves – this great empire that Charlemagne had created – was built, as we say in the book, on a foundation of lies. They weren’t just lies that the Franks told to other people, but lies they told themselves about their place in the world and the way that their empire was organised.

What we find so interesting is not just the things that happened but also the stories these people told themselves about those events.

How did tensions escalate after Charlemagne’s death?

David M Perry: Charlemagne had a lot of sons, and Louis was the only legitimate one who happened to survive. He had to work hard to build loyalty across his dad’s empire – it was not just handed to him. Some of that was done through persuasion and gifts, but some of it was achieved through violence and torture – in particular, the blinding of his nephew, Bernard of Italy, who then died. That caused great upset and upheaval in the court.

Louis tried to kick out the old guard and bring in the new. However, it turned out that the nobles comprising the old guard were still very powerful – and not super-thrilled about being kicked out. So there were these tensions right from the start. Then Louis remarried, taking as his wife a younger woman. The old guard weren’t thrilled with that, either, because she brought in her own power centres.

So Louis’ was not a stable reign, suffering from various internal problems, but it was also a moment at which the Franks started losing battles on the edges of the empire. And that was a whole other crisis of legitimacy.

Louis had to work hard to build loyalty across his dad’s empire – it was not just handed to him. Some of that was done through persuasion and gifts, but some of it was achieved through violence and torture

Matthew Gabriele: The stability and greatness of Charlemagne is often contrasted with the instability and fecklessness of Louis the Pious. But if you look at Charlemagne, he actually had a lot of problems governing, too. We talk about the coups of the 830s against Louis the Pious, but Charlemagne himself faced several attempted coups. One such attempt in 792 was led by his own eldest son, Pepin, known to later historians as Pepin the Hunchback, though he certainly wasn’t called that at the time.

It seems baked into the way the Carolingians ruled their empires that they had to rely upon powerful nobles around them. So when these nobles got angry about their loss of status, that was a real problem for the kings. And just because only a few coups were successful, that doesn’t mean that there weren’t other coups plotted and executed along the way.

What were Charlemagne’s grandsons trying to achieve at Fontenoy?

Matthew Gabriele: They were trying to decide the fate of the empire. Lothar was trying to beat his brothers into submission, and ideally to restore the empire as it had been under his father and his grandfather. Charles and Louis wanted something different: they wanted their own functional kingdoms within the empire.

They wanted to be free from the meddling of the emperor in their lands – Charles the Bald’s realm lying to the west, Louis the German’s to the east – and the right to act as independent Carolingians. They had, they argued, the same blood as Lothar’s flowing through their veins.

Looking back, this event became – especially for 19th-century historians – the moment that made France and Germany. It was seen as the final division of Charlemagne’s empire, and the birthplace of the modern state. But today it seems clear that the debate was much more nuanced. Nobody was arguing that Lothar shouldn’t be emperor, they were just arguing about what his role should be there.

It was an argument that was so important that they were willing to kill each other: brothers fought against brothers, and cousins, uncles and nephews were divided between the armies. These armies were camped across a field from each other, almost literally staring at each other. From all available sources, it seems that they exchanged emissaries, and decided that they would fight at 8 o’clock the following morning: Saturday 25 June 841.

How did the battle play out?

David M Perry: Lothar had a well-equipped army and probably slightly larger numbers of men, supported by Pepin’s forces. Charles the Bald had a quite large army, drawn from all over the Frankish heartland. Louis had a smaller army, but it was battle-hardened. Pepin and Charles met in the middle, Louis and Lothar met on the left, and there was a third division led by major nobles on the right.

We know little about what actually happened in the battle, but it’s clear that Lothar’s forces engaged Louis and were beaten back, and then Pepin’s forces in the centre collapsed. After that, there was a pursuit that resulted in horrific killing, but Lothar got away. He headed north-east to Aachen with the imperial treasury – and, once there, he spent wildly.

He launched a rumour campaign saying that one of his brothers was wounded and the other was dead, neither of which was true, and he immediately started rebuilding political support. This went on for some time. Eventually, Louis and Charles once again brought their armies together – and now they had clear military superiority, there was no dividing them again. So they brought Lothar to the negotiating table.

That brings us to the Treaty of Verdun in 843. What happened then?

David M Perry: I remember very clearly learning about the Treaty of Verdun while I was an undergraduate in my first medieval history class – and being taught a number of things that are not correct. I believed that there was a treaty – a document, like the Treaty of Versailles – that was written in Latin, German and French, and that it showed the division of France and Germany.

I don’t want to call out my undergraduate medieval history professor, because he’s the reason I’m talking to you today – he was a wonderful professor – but I remember learning these things, and they’re just not true. We have no document. There is absolutely nothing surviving from that meeting in 843 that lays out the division of the empire explicitly.

The year before the treaty, Louis and Charles travelled to the city of Strasbourg and undertook a public oath-taking. That was the moment when they came together and really made it clear they were going to sit down and talk until they reached an agreement – and where they confirmed their opposition to Lothar ruling the entire empire.

I believed that there was a treaty – a document, like the Treaty of Versailles – that was written in Latin, German and French, and that it showed the division of France and Germany... We have no document

As a result, Louis and Charles brought Lothar to the negotiating table, where they agreed that they would split up the empire into three parts. Then they had the problem that no one knew exactly how big the empire was, so they sent out nobles to do the accounting. Nine months later they met in Verdun, really just because it’s a convenient place where those nobles could come together.

We can figure out exactly how the empire was divided up only by looking at what happened over the next couple of years – where the brothers went, and which allies got which bits of land.

How did this civil war make medieval Europe?

Matthew Gabriele: The nostalgia for Charlemagne’s empire animated the vast majority of continental European history through to the 15th or 16th century. The kings of France as they emerged, the emperors in what is now Germany, the rulers of Italy, even the Christian Iberian rulers, all tried to tie themselves in some ways – and in many ways it was accurate – back to the Carolingians through some bloodline that emerged there.

In the 830s and 840s, there were just too many Carolingians. Whereas there had only been one Charlemagne, and then only one Louis the Pious, after that, there were too many legitimate children of Louis, and then their children, and then their children, all of whom had potential to rule. That tension, I think, was ultimately part of the problem that led to the civil war.

But then, in the centuries thereafter, there was a dream of empire – and especially the dream of a sense of election, of being God’s chosen people – that never disappeared. This view guides many of the actions that these men’s descendants took, including the push for the kingdom of France to expand beyond the Île de France and to reunite with the Carolingian bloodline.

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The desire to go on crusade in 1095, for example, was driven by the idea that these guys were all Franks and that they were fighting to reclaim Charlemagne’s claims to Jerusalem, which was another legend that evolved in the 10th and 11th centuries.

These medieval realms were different from what we think of as modern nation states such as France and Germany – they were different things aiming for different ends. Those modern states have sought justification for their formation in these events of the European Middle Ages, but the reality is different.

David M Perry: I just want to jump back into the oaths of Strasbourg, this famous moment on 14 February 842 for which we have a source written in Latin, but also French and German versions, because the men involved spoke all of these languages. Importantly, this was the first documented use of the French and German vernaculars.

It’s not that there was a French troop and a German troop, and a linguistic divide between different peoples. The whole point is that there were a lot of languages spoken across this huge land mass, and the men involved wanted to make sure that everyone understood everything.

In the Carolingian empire, people were able to speak all of these languages – that was just normal. It was a big, complicated empire, and people talked to each other. It was only 1,000 years later that this point became symbolic; a way of creating a narrative that linked the new French and German nation states and nationalistic identities back to that moment when the oaths were sworn.

Looking at how people in the 19th century viewed events in the Middle Ages is something that Matt and I do a lot. We can definitely say that what they read into the situation was not what was actually happening in the ninth century.

Did that mid-ninth-century civil war conclusively fracture the Carolingian empire?

Matthew Gabriele: The short answer is: yes. The empire was never stitched back together, though there were attempts to do so, especially in the later ninth century and the 10th century. In the first and second generations after the battle of Fontenoy, these attempts failed because of the bad blood that was generated there.

That wasn’t just because of the bloodiness of the battle itself, or the resentments on either side about who deserved what, but the way that the battle was pursued – the fact that Louis’ and Charles’s armies followed Lothar’s and cut down his men in their pursuit. It seems to have been a real massacre that people remembered for generations.

Matthew Gabriele is a professor of medieval studies at Virginia Tech and David M Perry is a journalist and historian. They are the co-authors of The Bright Ages: A New History of Medieval Europe (HarperCollins, 2022) and Oathbreakers: The War of Brothers that Shattered an Empire and Made Medieval Europe (Harper, 2024)

This article was first published in the January 2025 issue of BBC History Magazine