Louis the Pious's sons deposed him twice and fought a battle at Fontenoy, and the Treaty of Verdun split the empire into the lands that became France and Germany

In 817 Louis the Pious issued the Ordinatio Imperii at Aachen. It made his eldest son Lothar co-emperor and heir to the empire. His sons Pepin and Louis got subordinate kingdoms and could not make war or peace without Lothar. The act also said a dead brother's power "shall not be divided" among his sons. In 823 Louis's second wife, Judith, bore him a fourth son, Charles. Making room for Charles reopened the succession. A revolt in 830 failed. In 833 his three elder sons rebelled together; his army deserted him at the place later called the Field of Lies, and he surrendered on 30 June 833. He was made to do public penance at Soissons and was declared unfit to rule, then restored and recrowned at Metz. He died in 840. Lothar claimed the whole empire. Louis the German and Charles the Bald defeated him at Fontenoy on 25 June 841, leaving "a multitude of dead." On 14 February 842 at Strasbourg they swore alliance, one in a Romance language and one in a Germanic one. On 10 August 843 at Verdun the three brothers split the empire. Charles took the west, which became France. Louis took the east, which became Germany. Lothar took the middle strip from the Low Countries to Italy, with Aachen and Rome. That middle land, Lotharingia, was fought over by France and Germany into the 20th century.