The three-year Carolingian Civil War culminated in the decisive Battle of Fontenoy-en-Puisaye, also called the Battle of Fontenoy, fought at Fontenoy, near Auxerre, on 25 June 841. The war was fought to decide the territorial inheritances of Charlemagne's grandsons — the division of the Carolingian Empire among the three surviving sons of Louis the Pious. Despite Louis' provisions for succession, war broke out between his sons and nephews. The battle has been described as a major defeat for the allied forces of Lothair I of Italy and Pepin II of Aquitaine, and a victory for Charles the Bald and Louis the German. Hostilities dragged on for another two years until the Treaty of Verdun, which had a major influence on subsequent European history.

Although the battle is known to have been large, it was not well documented. Many historical sources are believed to have been destroyed after the war, leaving scant records from which to conjecture the numbers of combatants and casualties.