Battle of Fontenoy-en-Puisaye

On June 25, 841, the plain of Fontenoy-en-Puisaye became the scene of a major clash between the sons of Louis the Pious. Charles the Bald, king of West Francia, and Louis the German, king of East Francia, faced their elder brother, Emperor Lothair I, allied with Pepin II of Aquitaine. The battle was one of the bloodiest of the 9th century: according to chroniclers, it caused thousands of deaths on both sides. After a fierce fight, Charles managed to break through Lothair's right flank, while Louis, initially repulsed, regained the advantage thanks to the cohesion of his infantry. Lothair ends up retreating towards Aix-la-Chapelle. This military victory, acquired at the cost of enormous losses, sealed the Charles-Louis alliance and prepared the way for the political triumph which would lead to the Treaty of Verdun two years later.

On the death of Louis the Pious (840), Charlemagne's empire fell into a war of succession. Lothair I, as eldest and emperor, claims the entire imperial heritage. His younger brothers, Charles the Bald and Louis the German, refused this supremacy and concluded a military alliance. Pepin II, grandson of Charlemagne, for his part claims the kingship of Aquitaine and sides with Lothair. After several preparatory campaigns, the armies concentrated in Burgundy, a strategic region at the crossroads of communications between east and west. The Battle of Fontenoy is therefore the decisive confrontation which must decide between the heirs of the empire. The sources, notably the chronicle of Nithard (grandson of Charlemagne), emphasize the extreme violence of the fighting and the fratricidal dimension of the war.

The battle was fought on open ground, favorable for major infantry and cavalry maneuvers. Lothair took the initiative and concentrated his efforts on the troops of Louis the German, whom he initially repelled. But Charles the Bald succeeded in a decisive maneuver: he launched his men on the right flank of the imperial forces and managed to dislocate them. The fighting quickly becomes confused, with each side committing all its reserves. The heavy infantry and the Frankish cavalry clashed for a prolonged period, without elaborate maneuvers but in a brutal melee. The advantage ultimately turned in favor of the Charles-Louis coalition, thanks to their coordination and the exhaustion of Lothaire's troops. The emperor, seeing his army dislocated, withdrew to preserve the rest of his forces.

Fontenoy's victory, despite its appalling human cost, changed the political balance of the Carolingian Empire. It consecrates the alliance between Charles and Louis and permanently weakens Lothair. In the process, the two winners strengthened their ties by taking a mutual oath in Strasbourg in 842, a founding act in the linguistic differentiation between West Francia (former French) and East Francia (former German). Two years later, the Treaty of Verdun (843) ratified the division of the empire into three kingdoms: Western Francia of Charles, Eastern Francia of Louis and Lotharingia of Lothair. The battle therefore marks an irreversible turning point: the imperial unity desired by Charlemagne is definitively broken, opening the way to the creation of the kingdoms of France and medieval Germany.