On June 25, 841, at the Battle of Fontenay, Louis and Charles defeated their brother Lotharius I. An excerpt from the article:
"One of the most traumatic experiences of the ninth century, as Janet Nelson puts it, was the battle of Fontenoy, when the army of Charles the Bald and Louis the German met the troops of Lothar and of his nephew Pippin of Aquitaine on 25 June 841. The battle was significant in two respects: the heirs of Louis the Pious struggled against each other damaging the strength and reputation of the family, and a large number of Franks lost their lives on this day. Fontenoy was and is considered to have been the most bloody battle in Carolingian times – Agnellus of Ravenna gives an exaggerated total of 40,000 killed men in Lothar’s and Pippin’s army alone. Immediate responses make clear how shocked participants and observers were. Two narratives about the battle written by participants provide us with vivid impressions about the course of the confrontation: a poem by Angilbert and a history by Nithard, who had fought on opposing sides. The battle was mentioned in near contemporary annals (Annales Bertiniani, Annales Fuldenses etc.) as well as in histories, both north and south of the Alps (Nithard, Agnellus of Ravenna etc.)."