The Treaty of Verdun in 843 split up the Carolingian Empire. It helped to create a situation in which Germany and France became separate ethnic, cultural, and political entities.
After Charlemagne's death, the empire that he created and ruled split apart. After Charlemagne's son Louis (who succeeded him) died, Louis's three sons signed the Treaty of Verdun. It split up the Carolingian Empire between them, creating three kingdoms. One of these kingdoms was in the West and would form the core of France. One was in the East and would become Germany. By splitting the Carolingian Empire, the Treaty of Verdun created these separate kingdoms that would grow apart instead of staying together as part of one empire.