The Eastern and Western Schism of Rome
The Roman Empire might be the single most spacious area of conquest the world has known.. The Pax Romana (Roman Peace) included countries from the Orient to Europe. In the end, it became too unwieldy for a single man to rule. The empire cracked apart, leaving a heavily dichotomous Western and Eastern Rome.
The Empire was heavily agrarian but after the split, most of the good land went to the East, while the smaller countries of the West quarreled over food. The cohesiveness of the country fell apart after continuing raids by barbarians. The captive areas lost their sense of unity and started speaking the language of their conquerors, as well as worshipping their "pagan" gods. The citizens of Western Rome lost their traditional language, religion and pride.
In contrast, Eastern Rome did just fine. Blessed with fertile land and few enemies, Eastern Rome (later called Byzantium) thrived. Unlike its sister countries in the West, they retained the money and the resources to attract intellectuals and artists. They spoke Greek uniformly, as Christianity took over as the state religion.
The Constantine Empire carried on as the inheritor of Greek theological thinking, architecture, and language for hundreds of years, long after the Western Romans had reverted to paganism, cruelty and dust.
Her is the first part of th question, will write up the second part later