Which Byzantine emperor is known for reconquering much of the territory previously ruled by Rome?

The Byzantine emperor known for reconquering much of the territory previously ruled by Rome is Justinian. His reign, which lasted from 527 to 565 AD, is notable for ambitious military campaigns aimed at recovering lost provinces of the Western Roman Empire, particularly in North Africa and Italy. Justinian's military commander, Belisarius, played a crucial role in these efforts, successfully recapturing territories like the Vandal Kingdom in North Africa and parts of Italy, including Rome itself.

Justinian's ambitions extended beyond military conquests; he also aimed to restore the Roman Empire's former glory culturally and administratively. His reign is marked by significant legal reforms, including the codification of Roman law known as the "Corpus Juris Civilis," which had a lasting impact on legal systems in many countries.

The other figures mentioned had significant roles in different contexts: Constantine is known for founding Constantinople and establishing Christianity; Heraclius is recognized for his military campaigns against the Persians; and Theodosius is noted for making Christianity the state religion of the Roman Empire. However, none are primarily credited with the extensive reconquests that characterized Justinian's rule.