Several factors helped the Roman Empire expand so quickly, including military power, political stability, and economic growth.
1. Military power: Roman armies were well-trained and well-armed, and were able to quickly defeat opposing armies in battle.
2. Political stability: Unlike many other civilizations at the time, the Roman Empire had a stable system of government, with a system of checks and balances. This allowed them to make decisions and act effectively.
3. Economic growth: The Roman Empire had a robust economy based on trade and agriculture that allowed it to generate wealth and resources to support its armies and expansion.