An important aspect needed in an empire, is the ability for the empire to maintain control of its people. Another way for an empire to gain control, is through trade. If an empire has commodities to trade that other empires need, they have leverage over the other empires which can then be used to control them.
Ancient Empires like Rome gained power by conquering territories with their military. Their military was usually more advanced and more disciplined than the rest, allowing them to beat other competitors and empires. They consolidated power by enforcing their laws and extracting taxes from the conquered peoples. They maintained power by integrating the people into their society, whilst also letting them maintain a degree of autonomy. If they alienated the conquered people, they would have to constantly face a barrage of rebellions that would inevitably weaken them.
