Carolingian civil war
The Carolingian civil war was a violent crisis over the succession to the Carolingian Empire following the death of Emperor Louis the Pious in June 840 and lasting until the Treaty of Verdun in August 843. Louis's eldest son, the emperor Lothar I, laid claim to an undivided empire, while his younger brothers Louis the German and Charles the Bald sought large kingdoms of their own on the basis of previous divisions planned by the late emperor. Their nephew, Pippin II, laid claim to Aquitaine.
After Louis the Pious's death, Lothar moved immediately to disregard the division of the empire and secure for himself his father's imperial position. He reached out to Pippin in Aquitaine for support against Charles. Not content with Bavaria alone, Louis the German occupied the Rhineland laying claim to all of Germania. The first military move of the budding civil war was Lothar's campaign that forced Louis from the Rhineland in August. It increased Lothar's prestige in the east, but ended in an armistice. Lothar then marched to the Seine in September, laying claim to Neustria, before reaching a truce with Charles.
The climax of the war was the spring of 841. Charles crossed the Seine and Lothar crossed the Rhine. After forcing Louis to retreat, Lothar turned to face Charles. On 13 May, while Charles and Lothar were manoeuvring, Louis defeated Lothar's occupying army in Alemannia at the battle of the Ries. He joined up with Charles in early June. At the same time, reinforcments from Aquitaine arrived in both camps. The battle of Fontenoy took place on 25 June. Lothar and Pippin were routed, but the battle was not decisive.
In August 841, Lothar attempted to cross the Rhine but was turned back by Louis. Charles invaded Aquitaine before reoccupying the lands east of the Seine. Lothar quickly reoccupied them, forcing Charles back beyond the Seine, but was unable to recreate the support in West Francia that he had formerly had. In early 842, Louis crossed the Rhine. He and Charles swore the Oaths of Strasbourg on 14 February before marching north. They met Lothar's army on the banks of the Moselle in March 842. Lothar retreated towards Burgundy, conceding the imperial capital of Aachen.
Following their success, Louis and Charles divided the empire between themselves, leaving Lothar only Italy. This prompted Lothar to seek a negotiated settlement. By June 842, a new tripartite division had been agreed in principle. During the subsequent truce, Charles took control of Aquitaine and Louis crushed a rebellion in Saxony, which Lothar had supported. The final negotiations, begun by 120 commissioners drawn from all sides in October, were delayed by the need to formally describe and inventory the empire. This was completed in July 843. In August, the three brothers signed the Treaty of Verdun ending the war.