From Frankish Kingdom to Empire in the Middle Ages: Germans established many kingdoms on the land of the former Western Roman Empire, among which Frankish Kingdom was the most powerful. In 800, the Pope crowned Charles, marking the birth of Charlemagne's empire. In 843, Charlie's three grandchildren signed the treaty of verdun in Verdun, which divided the empire into three parts. These three countries later developed into France, Germany and Italy. 2. The expansion war of Charlemagne Empire and the formation of feudal system: Charlemagne Empire suffered huge losses in the decades-long expansion war, and many bankrupt farmers took refuge in feudal lords and became serfs attached to them. At the same time, Charlie gave most of the land he conquered to his deputies. In this process, the feudal system gradually formed. For example, there is a saying in medieval Europe that "the vassal of my vassal is not my vassal". Because the feudal hierarchy was formed by layers of land, monarchies and vassals at all levels had mutual obligations. Therefore, the vassal, that is, the retainer, only recognizes the Lord who is directly named as the Lord, and has no vassal relationship with the Lord who is named as the Lord himself. Therefore, the phenomenon that "the princes of my princes are not my princes" appeared in medieval Western Europe. This complicated relationship "caused confusion of rights and obligations" among feudal lords, and caused constant struggles and scuffles between feudal lords.