During this Early Middle Ages, Western Europe was united once for a short period under the rule of the Carolingian Empire. It began when Pepin the mayor fo the palace of Neurstria and Austrasia seized power and became the Frankish King. He was formally ointed the king by the Pope.
Upon Pepins death the new king became Charles the Great or Charlamagne. With a small army at his control he year after year expanded the size of his empire. First to Italy then to Spain and then into Germany. By the time he finished his expansions in 787 almost all of Western Europe was in his hands. Charlemagne ruled through local counts who ruled in his name. Ultimately his realm was too large to truly rule effectively, but his personal prestige insured his control. In 800 on Christmas Day Pope Leo, crowned Charlemagne emperor of Rome.
On the death of Charlemagne his empire rapidly dissolved. His son and grandson could not demand the personal loyalty that he had. His grandsons under the Treaty of Verdun divided the Empire into three seperate parts.