History of Roman Army Recruitment

[M. Caecilius]

The early-mid republic:
The legions were raised from an annual levy of eligible citizens who owned property, and the auxiliaries were supplied by allied cities and colonies. Every citizen was ready and willing to serve when the state required him to, and each campaign was merely a temporary interruption from civilian life. Nevertheless, during the Punic Wars mobilization rates were extremely high, with more than 1/3 of adult male citizens called up to the legions.

Just to add that there were periods when citizens were less than willing to serve. Livy reports several incidents at the annual levies, in particular where campaigns in less booty-rich and protracted wars were concerned. And then there is the long debate on whether the effects of longer campaigns were as serious as the Gracchi claimed. After all, many people stood to lose from unrewarding campaigns, considering only property owners were eligible (and then again, even before Marius there were times when proletarians and even slaves were allowed to join the legions - though mostly only in the direst of emergencies).

The reasons as to why military service had become unpopular in the late empire remains a mystery. At least on paper, service conditions were certainly no worse than during the principate: salaries were raised several times (or had the real worth gone down?), land grant was given on discharge, soldiers were allowed to marry and even live outside the fortress with their families. For most people in the countryside life wouldn't have become too comfortable either to make a military career seem too unattractive.
Or was it that potential fighting men found other professions where they could make a more generous income?

I wonder whether concessions made to soldiers - especially the one giving them the right to marry - were not part of a move to make service more appealing. Although long before that law was introduced, soldiers seem to have happily ignored the fact that they were not allowed to marry and had wives and children anyway... whatever the law thought.

As for the unpopularity of military service in the later empire, I would take into consideration that the risks had increased. There were far more wars, and increased raids, than there had been before, and hardly any involved making booty, after Trajan. As far as salaries go, they did go up, but as you say, the actual worth may have gone down. Rome was caught in something approaching hyperinflation if you are looking at the coinage of the time, and harsh (if futile) measures such as the Edict on Maximum Prices. If I recall correctly, Rome also shifted from paying soldiers (and gathering taxes) in kind rather than coinage at some point, but I'm a bit hazy about that.

Also, I wonder whether the recruitment of Germanic and other soldiers was the outcome of recruitment problems amongst the Empire's own citizens, or contributed to it, because those who felt they were "true" Romans no longer saw the army as prestigious enough if they had to share rank and status with barbarians. Not that citizenship meant as much after the Constitutio Antoniniana as it had before.