The longest-lasting achievement of the Byzantine Emperor Justinian was his

While Justinian did attempt to reconquer parts of the fallen Western Roman Empire, these military campaigns were largely temporary and did not lead to lasting control. The territories gained were often lost shortly after his reign, demonstrating that military achievements were not as enduring as his legal reforms.

Although Justinian worked to suppress the Arian heresy, his efforts in this religious conflict did not produce a lasting impact on the overall religious landscape of the region. The theological disputes continued long after his reign, illustrating that this achievement was not as significant or permanent as his legal codification.

Justinian's establishment of Constantinople as the capital had considerable significance for the Byzantine Empire; however, the city had already been established by Constantine the Great. Therefore, while he strengthened its status, this action does not represent a unique or lasting achievement attributable solely to Justinian.

Justiniano's naval forces did experience periods of strength and dominance in the Mediterranean, but like his military campaigns, these achievements were often fleeting. Control over the sea routes fluctuated with time, making it less of a lasting contribution compared to his legislative accomplishments.