Explanation: Other than the obvious points of “Ancient Rome was very warlike” and “The Roman Republic had a ridiculous capacity to recruit and motivate new troops even after horrific losses”, it might also be added that the Legions by the time of the Roman Empire had also become very multipurpose tools! Everything from construction to policing to provincial bureaucracy to mining to tax-collecting could be undertaken by a Roman legion, should the local region lack institutions of its own for said purposes!

Except for the Basque. No one conquers the Basque.