Who was the emperor who retook much of the Western RomanEmpire that had been lost

It was Justinian I, the emperor of the Eastern Roman Empire, or Byzantine Empire, who ruled from 527 to 565 AD. He is one of the most important Roman emperors of history. After the fall of Rome, the barbarian people of the Goths had taken Italy. Justinian I launched a successful military campaign to retake Italy from the Goths (535-554 AD).

Flavius Belisarius