Final answer:
Justinian extended Byzantine power by reconquering western provinces with military campaigns led by his general Belisarius, establishing military governance through exarchs, and battling the Persians.
Step-by-step explanation:
Emperor Justinian was dedicated to expanding the Byzantine Empire and restoring Roman territories. The initiative to reconquer lost provinces in the west was pivotal to his reign. Justinian sent his general Belisarius to North Africa, where he defeated the Vandals in a year.
Next, Belisarius was dispatched to Italy, eventually capturing Rome and other major cities. Justinian’s campaigns also included the successful recapture of parts of Spain from the Visigoths. However, these conquests put a strain on the empire's resources, especially in the face of constant Persian conflicts.
Despite the temporary expansion, the Byzantine hold on these territories was relatively short-lived after Justinian's death.
Justinian's efforts to establish control included the appointment of exarchs in reconquered territories, merging civil and military authority to ensure stronger governance.
Although these measures extended the empire’s control for a time, long-term maintenance of these territories was challenging due to ongoing military pressures, particularly from the Persians in the east.