What were the main opponents of the Byzantine Empire under Justinian?

The main opponents of Byzantium during this period of time were: Slavs, Visigoths, Ostrogoths, Avars. Nevertheless, the Byzantine Empire during the reign of Emperor Justinian was very strong and powerful. Thanks to the rule of Justinian, vast territories (North Africa, Illyria, Italy) were returned to the empire, which were captured by the barbarians in the 5th century. But still, Byzantium occupied vast territories, which meant the presence of numerous external enemies with whom it was necessary to fight.