Question

How did the Byzantine Empire form?

Answer 1

1. It grew out of the Roman Empire. In fact, you could legitimately argue that it was the same empire. 
2. In the 3rd Century AD, the Roman Empire was divided into two halves, East and West. The Roman Emperor Constantine founded and built up the city of Constantinople as the Eastern capital of the Empire. It soon grew into a city that was bigger and more prosperous than Rome itself. 

Answer 2

The Byzantine Empire was form after the Roman Emperor Constantine was able to build the Byzantine Empire after he was able to seize the Eastern part of Rome after it was stolen away from his from his brother-in-law. Constantinople was the capital of the Byzantine Empire