The Treaty of Verdun, signed in August 843, was the first of the treaties that divided the Carolingian Empire into three kingdoms among the three surviving sons of Louis the Pious, who was the son of Charlemagne.
The treaty, signed in Verdun-sur-Meuse, ended the three-year Carolingian Civil War.
Following Charlemagne's death, Louis was made ruler of the Carolingian empire. During his reign, he divided the empire so that each of his sons could rule over their own kingdom under the greater rule of their father.
Lothair I was given the title of emperor but because of several re-divisions by his father and the resulting revolts, he became much less powerful.
When Louis the Pious died in 840, his eldest son, Lothair I, claimed overlordship over the entirety of his father's kingdom in an attempt to reclaim the power he had at the beginning of his reign as emperor.
He also supported his nephew, Pepin II's claim to Aquitaine, a large province in the west of the Frankish realm. Lothair's brother, Louis the German, and his half-brother Charles the Bald refused to acknowledge Lothair's suzerainty and declared war against him.
After a bloody civil war, they defeated Lothair at the Battle of Fontenay in 841 and sealed their alliance in 842 with the Oaths of Strasbourg which declared Lothair unfit for the imperial throne, after which he became willing to negotiate a settlement.