Why in 325 CE did Constantine move his capital from Rome to Byzantium which he renamed Constantinopl?

Answer 1

Conctantine did not move the capital in 325. Constantinople was inaugurated in 330.

Constantine I (or the Great) did not move the capital of the Roman Empire form Rome to Constantinople. Rome had already ceased to be the capital of the empire under a previous emperor, Diocletian. Diocletian had designated Nicomedia (in north-western Turkey) as the imperial capital of the eastern part of the Roman Empire and Milan (in Italy) as the imperial capital of the western part of the western part of the empire. What Constantine did was basically to move the imperial capital from Nicomedia to the nearby Byzantium (69 miles to its west), which he redeveloped, renamed Constantinople (city of Constantine) and inaugurated in 330.

Ancient historians did not actually clarify why Constantine did this. One reason might have been that Nicomedia was associated with Diocletian who had unleashed the Great Persecution of Christians.

A highly likely reason was Propaganda, which Constantine was very good at. He even invented a fictitious descent from the family of a previous emperor to give legitimacy to his quest for power. A new capital named after himself would give him further prestige and act as a symbol of the new dawn for the empire his rule would usher in. Besides being named after Constantine, the city was also given titles such as "The New, Second Rome,' 'Eastern Rome,' 'Alma Roma,' and 'Roma Constantinopolitana.' Moreover, Byzantium had seven hills like Rome. This created a similarity with Rome and justified the titles mentioned above.

Originally Constantine considered Sirmium (in modern day Serbia) for his new capital. Then he opted for Byzantium. Various factors may have helped this choice. This city was on the route between Europe and Asia like Nicomedia, but it was in a better strategic position. The roads from south-Western Europe to Asia converged there. It was on the Bosporus, the strait between Europe and Asia, and between the Mediterranean and the Black Sea. Therefore, it also controlled the naval route between these two seas. It was also easy to defend. It was on a promontory into the Bosporus. Therefore, it had water on three sides. The Golden Horn, an inlet of the Bosporus, provided a coast which went deeper inland on the northern side and, equally importantly, a very good harbour. The hilly terrain helped to defend the part of the city which was not on the seafront or by the Golden Horn.

Answer 2

Why in 325 CE did Constantine move his capital from Rome to Byzantium, which he renamed Constantinople

Rome was too vulnerable to attack from Germanic tribes