The Roman army was the greatest fighting machine the ancient world produced. The Roman Empire depended on soldiers not just to win its wars, defend its frontiers and control the seas but also to act as the engine of the state. Roman legionaries and auxiliaries came from across the Roman world and beyond. They served as tax collectors, policemen, surveyors, civil engineers and, if they survived, in retirement as civic worthies, craftsmen and politicians. Some even rose to become emperors.