Who did Justinian I reconquer?

Answer 1

Justinian I (or the great) did not want to reconquer anything with his enemies. That would have been a contradiction in terms. Justinian I carried out his reconquests exclusively with his imperial army.

Justinian I wanted to 'restore' the Roman Empire by reconquering the lands lost by the western part of the Roman Empire as a result of the Germanic invasions.

Justinian's army defeated the Vandals in Africa (Vandalic War, 533-534) retaking the area of the Vandalic kingdom (northern Tunisia and eastern Algeria). Then it had to fight a rebellion by the local Berbers until 548 to take southern Tunisia and western Libya. Justinian wanted to restore Africa as it had been under the Romans. The religion of the Vandals, Arian Christianity, was persecuted. Many Vandals fled Africa and some went west, to Algeria, and integrated with the local Berbers. The remaining Vandals were shipped to the east and enlisted in the imperial army. The Vandal ethnic group disappeared.

Justinian I attacked the Ostrogoths in Italy, starting the Gothic War (535-54) which he won. However, this was a highly costly victory. Nineteen years of fierce fighting left some areas of northern Italy devastated and depopulated. The war depleted the resources of the empire. This and the impoverishment of Italy made it impossible for the Byzantines to oppose the invasion of northern Italy by the Lombards fifteen year later (569-72). The Lombards also took over areas of central and southern Italy. The remaining Byzantine possessions in Italy were in part of central Italy and in Apulia, Sicily, Sardinia and Corsica.

Justinian managed to retake most of southern Spain along from the Visigoths on 559. The Visigoths retook this area in 624.