On August 10, 843, at the Treaty of Verdun, brothers Lotharius I, Louis the German, and Charles the Bald divided Charlemagne's empire. From the article:
"Treaty of Verdun: 843
The Treaty of Verdun 843
The mighty Carolingian empire, with its great history of skillful rulers, was divided by the Treaty of Verdun. Its rise was begun by Pepin of Herstal and his son, Charles Martel. Their family gained power during the demise of the Merovingian Dynasty. During this time, Pepin of Herstal, Charles Martel, and his grandson, Pepin the Short, became the most powerful men in the Frankish state. In 751 AD, Pepin the Short disposed of the reigning Merovingian king and became the king of the Franks. He also received a special anointment by the pope. The greatest emperor, however, was Charlemagne. Charlemagne was crowned the Holy Roman Emperor. With the death of Charlemagne, his son Louis the Pious inherited the throne.
Around the year of 840 AD after Louis the Pious' death, warfare broke out between his three sons. Lothair, being the eldest, was to receive most of the land. Wanting a piece of this wealthy inheritance, Charles the Bald and Louis the German allied against their half brother, Lothair, for his land and title. After Lothair was defeated at Fontenoy and Aix-la-Chapelle, he asked for peace. The war was officially concluded with the Treaty of Verdun. Charles the Bald won the kingdom of the West Franks. Louis the German received the East Franks, and Lothair gained the middle stripe-which includes Lorraine and the Papal States. He also kept the title of Holy Roman Emperor.
The kingdom of the West Franks was harassed by the Vikings, who ravaged their lands. There were also problems in governing the province of Aquitaine. Still, Charles the Bald remained in control. After his death, the kingdom of the West Franks was passed to his son, Louis II the Stammer.
Louis the German ruled the kingdom of the East Franks. When he died, he partitioned his kingdom to his sons. Lotharingia was the least stable of the three kingdoms that were divided by the Treaty of Verdun. It also was partitioned among Lothair's three sons, who raged war against each other, and eventually brought it to ruin.
The Treaty of Verdun was one of the most important treaties of Europe. It divided Charlemagne's vast empire and laid the foundations for what would become the independent states of France and Germany."