As the Romans

What cries out for explanation is not the Romans’ militaristic character or psychic aggression, but why in a world that was universally violent the Romans were so consistently more successful than their enemies and rivals. The basic answer to that has little to do with superior tactics or even with better military hardware; it has much more to do with boots on the ground. In its early centuries at least, standard Roman practice, unique in the ancient world and most of the modern, was to turn those it had defeated into Roman citizens and to convert erstwhile enemies into allies and future manpower. It was an empire built – as those desperate refugees on the Danube must have hoped, long after the policy had ceased to be feasible – on the extension of citizenship and the incorporation of outsiders.

Beard, Mary. "Why Ancient Rome Matters to the Modern World," The Guardian, Accessed on October 4, 2015, http://www.theguardian.com/books/2015/oct/02/mary-beard-why-ancient-rome-matters