Recruited at the age of seventeen years and serving for at least twenty five, one of the keys to Roman success was the world's first professional soldier.
Five thousand of these heavily armed infantry made up a legion. Their perfect discipline gave them long term superiority over enemies who were not prepared to undergo the same rigorous training.
Like the battleships and aircraft carriers of a modern navy the legions were usually stationed some way behind the empire's frontier. In many of Rome's later wars the actual fighting was done by auxiliary troops raised from the tribes who lived along the empire's borders.
As the ultimate weapon there were times when the legions suffered from inactivity. They also proved to be vulnerable when attacked on the march or when surrounded and unable to manouevre.
However, the worst damage to Rome's armies was done during civil wars between rival emperors. Lack of a system for ensuring the smooth transfer of power proved to be the empire's greatest political weakness.