The numerous wars of Justinian were partly offensive and partly defensive. The former were carried on against the barbarian Germanic states of western Europe; the latter were directed against Persia in the East and the Slavs in the north.
The main forces were directed to the west, where the military activities of the Byzantine army were crowned with triumphant success. The Vandals, the Ostrogoths, and to some extent the Visigoths were forced into subjection to the Byzantine emperor. The Mediterranean Sea was almost converted into a Byzantine lake. In his decrees Justinian called himself Caesar Flavius Justinian the Alamannicus, Gothicus, Francicus, Germanicus, Anticus, Alanicus, Vandalicus, Africanus. But this outer splendor had its reverse side. The success was attained at a price too dear for the Empire, for it involved the complete economic exhaustion of the Byzantine state. In view of the fact that the army was transferred to the west, the east and the north remained open to the attacks of the Persians, Slavs, and Huns.
The principal enemies of the Empire, in Justinian's opinion, were the Germans. Thus the German question reappeared in the Byzantine Empire during the sixth century, with this difference only: in the fifth century the Germans were attacking the Empire; in the sixth century it was the Empire that pressed upon the Germans.