In 843, the three surviving grandsons of Charlemagne, Lothair, Louis the German, and Charles the Bald, signed the Treaty of Verdun, ending three years of civil war by splitting the Carolingian Empire into West Francia, Middle Francia, and East Francia. The rough boundaries anticipated the future shapes of France and Germany, and the disputed middle kingdom became a fault line of European conflict for over a thousand years.
Charlemagne had unified most of Western Europe under one crown, but Frankish tradition demanded that a father's lands be divided among his sons. When Louis the Pious died in 840, his three sons plunged the empire into war, culminating in the bloody Battle of Fontenoy in 841, where tens of thousands of Frankish warriors reportedly died fighting each other.
Exhausted, the brothers negotiated at Verdun. Charles the Bald received the western lands, roughly the outline of modern France. Louis the German took the east, the seed of Germany. Lothair, the eldest, kept the imperial title along with a long, awkward strip of territory running from the Low Countries down through Burgundy to Italy, a kingdom with no real geographic or ethnic coherence.
That middle strip, Lotharingia, proved impossible to hold together and fragmented within a generation. Its remnants, Alsace-Lorraine, the Low Countries, northern Italy, became the contested borderlands fought over in wars from the Middle Ages through 1945. Historians still point to Verdun as the moment a single Frankish civilization began splitting into recognizably French and German identities.