The Men from the Ranks
The main supply for the centurionate of the legions came from the ordinary men from the ranks of the legion. Though there was a significant number of centurions from the equestrian rank.
Some of the late emperors of the empire prove very rare examples of ordinary soldiers who rose all the way through the ranks to become high-ranking commanders. But in general the rank of primus pilus, the most senior centurion in a legion, was as high as a ordinary man could go.
Though this post brought with it, at the end of service, the rank of equestrian, including the status – and wealth ! – that this elevated position in Roman society brought with it.
The ordinary soldier’s promotion would start with the rank of optio. This was the assistant to the centurion who acted as a kind of corporal. Having proven himself worthy and earned promotion an optio would then be promoted to being a centurio.
However for this to happen, there would have to be a vacancy. If this was not the case he might be made optio ad spem ordinis. This marked him out by rank as ready for the centurionate, merely waiting for a position to become free. Once this happened he would be awarded the centurionate. But, there was further division between the seniority of centurions. And as a newcomer, our former optio would start on the lowest rung of this ladder.
With their being six centuries in each cohort, each regular cohort had 6 centurions. The centurion commanding the century most forward was the hastatus prior, the one commanding the century immediately behind his, was the hastatus posterior. The next two centuries behind them were commanded respectively by the princeps prior and the princeps posterior. Finally the centuries behind these were commanded by the pilus prior and the pilus posterior.
Seniority between the centurions was most likely such that the pilus prior commanded the cohort, followed by the princeps prior and then the hastatus prior. Next in line would be the pilus posterior, followed by the princeps posterior and finally the hastatus posterior. The number of his cohort was also part of a centurion’s rank, so the full title of the centurion commanding the third century of the second cohort would be centurio secundus hastatus prior.
The first cohort was the most senior in rank. All its centurions outranked the centurions of the other cohorts. Though according to its special status, it had only five centurions, their being no division between pilus prior and posterior, but their role being filled by the primus pilus, the highest ranking centurion of the legion.
Under the republic the equestrian class supplied the prefect and the tribunes. But generally there was not a strict hierarchy of different posts during this era. With the increased numbers of the auxiliary commands becoming available under Augustus, a career ladder emerged with various posts available to those of equestrian rank.
The main military steps in this career were:
praefectus cohortis = commander of an auxiliary infantry
tribunus legionis = military tribune in a legion
praefectus alae = commander of an auxiliary cavalry unit
With both the prefect of an auxiliary cohort and the prefect of the cavalry, those commanding a millaria unit (roughly a thousand men) were naturally deemed senior to those commanding a quingenaria unit (roughly five hundred men). So for a praefectus cohortis to move from command of a quingenaria to a millaria was a promotion, even if his title would not actually change.
The various commands were held one after another, each one lasting three or four years. They were generally given to men who had already gained experience in civilian positions of senior magistrates in their home towns and who were perhaps in their early thirties. Commands of a cohort of auxiliary infantry or a tribunate in a legion were usually granted by the provincial governors and hence were largely political favours.
Though with the award of cavalry commands it is likely that the emperor himself was involved. Even with some of the commands of millaria auxiliary infantry cohorts it appears that the emperor made appointments.
Some equestrians went on from these commands to become legionary centurions. Others would retire to administrative posts. There was however a very few enormously prestigious posts open to experienced equestrians. the special status of the province of Egypt meant that the governor and legionary commander there could not be a senatorial legate. It hence fell to an equestrian prefect to hold command of Egypt for the emperor.
Also the command of the praetorian guard was created as a post for equestrians by emperor Augustus. Though in later days of the empire naturally the increasing military pressures began to blur the lines between what was reserved strictly for the senatorial class or for equestrians. Marcus Aurelius appointed some equestrians to legionary commands simply by making them senators first.
The Senatorial Class
In the changing Roman empire under many reforms introduced by Augustus the provinces continued to be governed by senators. This left open to the senatorial class the promise of high office and military command.
Young men of the senatorial class would be posted as tribunes to earn their military experience. In every legion of the six tribunes one position, the tribunus laticlavius was reserved for such a senatorial appointee.
Appointments were made by the governor/legatus himself and hence were among the personal favours he make to the young man’s father.
The young patrician would serve in this position for two to three years, beginning in his late teens or early twenties.
Therafter the army would be left behind for a political career, gradually climbing the steps of the minor magistracies which could last for about ten years, until finally the rank of legionary commander could be reached.
Before this however, usually would come another term of office, most likely in a province without legions, before reaching the consulate.
The province of Egypt, so important for its grain supply, remained under the emperor’s personal command. But all the provinces with legions within them were commanded by personally appointed legates, who acted both as army commanders as well as civil governors.
After having been consul an able and reliable senator might be appointed to a province containing as many as four legions. The length of service in such an office would generally be for three years, but it could vary considerably.
Almost half of the Roman senate was required to at some time serve as legionary commanders, indicating just how competent this political body must have been in military matters.
The length of office for able commanders however increased with time. By the time of Marcus Aurelius it was well possible for a senator of great military talent to hold three or even more successive major commands after he had held the consulate, after which he might progress onto the emperor’s personal staff.