The Roman Empire was one of the few major Mediterranean Empires, officially starting in 27 BCE, and ending in 476 CE. At it's height in 117 CE, under the reign of it's 13th emperor, Emperor Trajan, it had spread all across the Mediterranean Sea, and was one of the most powerful and influential empires of it's time. Unfortunately, in 312 CE, the emperor at the time – Emperor Constantine – decided he wanted to take the empire in a new direction. He announced that he had "had a vision" and declared that he was now a Christian, and that Christianity, a minority religion at the time, is now the religion of the land. This caused much backlash throughout the entire empire, and it only got worse after that. There are many other reasons why Rome fell, but most of them are consequences or subsets of these three: overexpansion, political instability caused by Christianity, and foreign invasion by the Vandals and Visigoths.
The Roman Empire, beginning as just a small city-state in what is now modern-day Italy, expanded it's territory mainly through military aggression and invasion, and would continue to do so for another 400 years. In fact, in 146 BCE, Rome had conquered Greece due to the Greeks forming a military alliance with the Carthaginians, a civilization of which they were at war with at the time. Annexing Greece gave the Romans one of their first major land gains. However, as Rome continued to aggressively expand it's territory, the amount of land it had control over became more of a burden than a blessing, mainly due to costs – the sheer amount of land they owned costed them a fortune to control, defend, and maintain. The military and infrastructure budgets both suffered as a result. In one ancient Roman text, written by a citizen, named Florus, who wanted to record the empire's history, said, "Again, what brought the servile wars upon us except the excessive size of our establishments?" (Florus).