In the older days of Roman history the fighting forces had been a "citizen army," called out for so long as it was needed, and levied from full and true Roman citizens. In the imperial times with which we are here dealing it had become a standing army. Soldiering was a profession, for which the men volunteered, and, so far as Roman citizens were concerned, it was now seldom, if ever, the case that military service required to be made compulsory on their part.