Upvote:0
The manpower drain was a problem in late republic, when soldiers where mostly small farmers. Former legionaries are expected to buy their equipment and partially sustain themselves. At the end of campaign they wold receive land to be compensated. Marius permitted the recruitment of capit sensi, or proletarians, citizens without posses, most of them urban plebeian, unable to afford this by giving them equipments and paying salaries.
But that drain was not about reduced labour output, as Rome had a slave based economy and roman citizens were mostly "improductive", but about the availability of citizens in age. The state paid army permitted the immense growth of the legions in early empire.
Upvote:4
Dando-Collins may be right, but not for the period you specify in your comment.
As I mentioned, the nature of the Roman military changed over its history. In the Kingdom and Republic, the military was mostly levied from the population as and when needed, and typically only for a single campaigning season. Sometimes a legion could be raised as a standing army, particularly to occupy invaded territory, but there was a 6 year limit on service by any individual. It was probably this period that Dando-Collins is talking about - a legion would be raised from a local population to deal with a threat or engage in a war and then disbanded en masse once victorious.
Between Marius and Augustus (late Republic to early Empire), the military was transitioning into a professional volunteer army. However, the civil wars leading up to Augustus' reign saw the number of legions bloat incredibly, with many soldiers being conscripted on both sides of the fight. Even after disbanding the defeated forces, Augustus had 50 legions!
Eventually, Augustus pared down the legions to 28, all of them volunteers, and most of them became long-lived units continuing for at least a couple of centuries. As legion service was now 25 years, it wouldn't make sense to recruit and disband en masse.