Answer:
The Roman Empire was successful because they were dominant in war, and they had a good economic system. The Romans were smart, ambitious, creative people. They were practical as well, meaning that they could work together to create a successful empire. They were set on getting anything they craved, meaning that they were also aggressive.
NOTE: An important thing for ANY empire, is water. Water is something that is very prized. If you have a good water source and get plenty of water from it, your empire will be very successful. Even in modern society, the powerful cities are the ones that get plenty of water.