The first really famous Byzantine Emperor came to the throne in 527. Justinian (right) was the nephew of Justin I, two emperors removed from Zeno. In nearly 40 years on the throne, Justinian, mainly through his famed general Belisarius, reconquered much of what had been the Western Roman Empire.

He started with the Sassanid Empire, dismissing them as a threat for the last time in 532. Targeting the Vandals, Belisarius took an army of 15,000 men into North African and "liberated" it, establishing an African prefecture at Carthage. During the next five years, Belisarius and his troops conquered the Ostrogoths and rolled back their occupation of Italy.

In the same year that Rome found a new (and, at the same time, old) ruler, 540, Belisarius had to sail back eastward and re-engage with the Persians. A plague outbreak stalled the fighting between the two powers, which ended with another truce in 545.

At the same time, the Ostrogoths had thrown off the Roman yoke and attempted to reassert their authority in Italy. They were successful for a time but lost their hold on the area again, thanks to the determination of the Roman armies. This time, it was the general Narses who was able to turn the tide in Justinian's favor, defeating the Ostrogoths under Titila at Busta Gallorum in 552. Two years later, the Romans had to contend with an invasion of the Frankish army and were again successful.

Rounding out Justinian's reconquests was a bit of Hispania. By 555, the amount of land controlled by the Roman Empire (for it wasn't until later that this entity was called the Byzantine Empire) approached what it had been in the heyday of expansionist emperors like Trajan.