system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why was the Roman Army so effective/unbeatable?
gpt
They were well-organized, well-disciplined and well-equipped, but far from unbeatable. They sufferred defeats from both the northern 'barbarians' and the more civilized nations around the Mediterranean. By the time the bar of entry to the military was lowered to a 'head count' (prior to some reforms military service was a duty to free, land-owning men who had to supply their own equipment etc) the Roman state was wealthy enough to field permanent, professional armies who could march, drill and practice all-year round.

Much of their fighting style was formation based, and since the strength of a formation requires it to be intact they benefited greatly from having disciplined men who wouldn't crack under the pressure of a heated battle. The Romans also made good use of auxiliary troops from the nations they conquered/extracted tribute from, so if they were fighting one Gallic tribe chances were they had e.g Gallic cavalry support from another and so on.

The extensiveness of the Roman roads (all roads lead to Rome, remember?) and trade made it a logistical possibility for their armies to be supported even in remote regions where others might've struggled to feed and maintain the equipment of thousands of men.

**In essence, they had a big manpower pool to draw recruits from, they could afford to equip them and train them extensively and their infrastructure allowed them to deploy them across vast distances.**