In the Treaty of Verdun of 843 the three surviving sons of Louis the Pious divided his territories into three kingdoms. The eldest son, Lothar, had waged war against his brothers since the death of their father in 840. After his defeat at the Battle of Fontenay (841) and his brothers' alliance sealed in the Oath of Strasbourg, Lothar was willing to negotiate.
Each of the brothers was already established in one kingdom - Lothar in Italy, Louis the German in Bavaria, and Charles the Bald in Aquitaine. Lothar received the central portion of the empire - what later became the Low Countries, Lorraine, Alsace, Burgundy, Provence, and Italy - and the imperial title as an honor without more than nominal overlordship. Louis the German received the eastern portion, much of what later became Germany through the shape of the Holy Roman Empire. Charles the Bald received the western portion, much of what later became France.
Though often presented as the beginning of a devolution or dissolution of Charlemagne's unitary empire, it in fact reflected the continued adherence to the Frankish idea of a partible or divisible inheritance rather than primogeniture.