There were three of these grandsons, and, when their struggle had left them thoroughly exhausted, they divided the empire into three. Their treaty of Verdun (843) is often quoted as beginning the modern kingdoms of Germany, France, and Italy. The division was in some sense a natural one, emphasized by differences of language and of race. Italy was peopled by descendants of the ancient Italians, with a thin intermingling of Goths and Lombards; France held half-Romanized Gauls, with a very considerable percentage of the Frankish blood; while Germany was far more barbaric than the other regions. Its people, whether Frank or Saxon, were all pure Teuton, and still spoke in their Teutonic or German tongue.
The Franks themselves, however, did not regard this as a breaking of their empire. They looked on it as merely a family affair, an arrangement made for the convenience of government among the descendants of the great Charles. So firm had been that mighty hero’s grasp upon the national imagination, that the Franks accepted as matter of course that his family should bear rule, and rallied round the various worthless members of it with rather pathetic loyalty, fighting for them one against the other, reuniting and redividing the various fragments of the empire, until the feeble Carlovingian race died out completely.
It is thus evident that there was a strong tendency toward union among the Franks. But there was also an outside influence to disrupt their empire. Charlemagne had not carried far enough their career of conquest. He subdued the Teutons within the limits of Germany, but he did not reach their weaker Scandinavian brethren to the north, the Danes and Norsemen. He chastised the Avars, a vague non-Aryan people east of Germany, but he could not make provision against future Asiatic swarms. He humbled the Arabs in Spain, but he did not break their African dominion. From all these sources, as the Franks grew weaker instead of stronger, their lands became exposed to new invasion.Source