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

While Justinian made significant military efforts to reconquer parts of the Western Roman Empire, these gains were often short-lived and did not establish lasting control. The territories regained were frequently lost soon after his reign, making this achievement less enduring than his legal reforms.

Justinian did take measures against the Arian heresy, particularly in Italy, but this was a religious and political struggle rather than a lasting accomplishment. The Arian controversies persisted long after Justinian's death, showing that this achievement did not create a permanent resolution.

The relocation of the capital to Constantinople (formerly Byzantium) was established by Emperor Constantine, not Justinian. While Justinian strengthened and expanded the city, the decision to move the capital was made long before his reign, meaning this is not a unique achievement attributable to him.

Though Justinian did invest in a navy, its effectiveness varied, and control over the Mediterranean was often contested. This military achievement did not have the same lasting impact as his codification of laws, which has continued to resonate throughout legal history.