Rome had established control over its vast territory thanks to the strength of the citizen-soldiers of the Republic. As described in the last chapter, however, the republican military system declined after the Punic Wars as the number of free, economically independent Roman citizens capable of serving in the army diminished. By the first century, most Roman soldiers became career soldiers loyal to a specific general who promised tangible rewards rather than volunteers who served only in a given campaign and then returned home to their farms.
Perhaps the most important thing Augustus did besides establishing the principate itself was to reorganize the Roman legions. He created a standing professional army with regular pay and retirement benefits, permanently ending the reliance on the volunteer citizen - soldiers that had fought for Rome under the republic. Instead, during the empire, Legionaries served for twenty years and then were put on reserve for another five, although more than half died before reaching retirement age. The major benefits of service were a very large bonus paid on retirement (equivalent to 13 years of pay!) and land: military colonies spread across the empire ensured that a loyal soldier could expect to found a prosperous family line if he lived that long.
Service in the army was grueling and intense. Roman soldiers were expected to be able to march over 20 miles in a standard day's march carrying a heavy pack. They were subject to brutal discipline, up to and including summary execution if they were judged to have been derelict in their duties - one of the worst was falling asleep on guard duty, punishable by being beaten to death by one's fellow soldiers. Roman soldiers were held to the highest standards of unit cohesion, and their combat drills meant they were constantly ready for battle.
Starting in the Augustan period, the essential division in the Roman military was the legion, a self-sufficient army unto itself that could be combined with other legions to form a full-scale invasion force but could also operate on its own. During the Augustan period, each legion consisted of:
- The Legion: 5,400 infantry and specialists with 120 cavalry.
- Each legion was subdivided into 10 cohorts of 480 men each led by 6 centurions, veterans who led the cohorts in combat.
- Each cohort had six centuries of 80 men with each personally led by one of the centurions.
- Each century divided into 8 squads of 10 men, referred to as "tentmates" since they lived, worked, ate, and trained together.
- In addition, there were hundreds of specialists, from engineers to smiths and cooks. Each legion was thus self-sufficient as it traveled and could be expected to operate within hostile territory for long periods if necessary.
Each legion was led by a legionary legate, usually a powerful noble appointed by the imperial government or the emperor himself. These legates were often politicians rather than soldiers, meaning that the key figures in actual battle were the centurions, each of whom had earned his position through exemplary service. Perhaps most important of all was the lead centurion, the First Spear, who dictated tactics on the field.
The legions were made up of Roman citizens, but not all members of the Roman military were citizens. Instead, as numerous as the legions were auxiliaries: Roman subjects (e.g. Celts, North Africans, Syrians, etc.). who nevertheless served the empire. The auxiliaries were divided into cohorts of infantry and alae ("wings") of cavalry. In comparison to the infantry-focused Roman legions, the auxiliaries tended to vary their arms - auxiliaries could be slingers and archers as well as foot soldiers and cavalry. They tended to serve as scouts and support for the legions as well as engaging in combat in their own right. As of 23 CE they numbered about 150,000 men, which was the same as the legions at the time. The emperor Claudius rewarded 25 years of service with citizenship; by the early second century, all auxiliaries gained citizenship on discharge.
A key legion that stood apart from the rest of the military was the Praetorian Guard, whose major job was defending the emperor himself, followed in priority by the defense of Italy and the city of Rome. The Praetorian Guard started as nine cohorts of 480 men, but later each cohort was grown to 1,000 men. The terms of service in the Praetorian Guard were very attractive: 16 years instead of 25 and pay that was significantly higher (this was a necessity: emperors started with Claudius knew that they were vulnerable to the Praetorians and needed to keep them happy and loyal). Not surprisingly, Praetorians were recruited from veteran legionaries. They did not simply serve the emperor in the city of Rome, instead actively campaigning both when defending Roman territory from invasion (which became an increasing problem by the fourth century CE), and with the emperor while on campaign.
The army was important in integrating provincial subjects into Roman culture. A soldier recruited from the provinces had to learn Latin, at least well enough to take orders and respond to them. Auxiliaries served with men from all over the empire, not just their own home regions, and what each soldier had in common was service to Rome. Commanding officers were often from the Italian heartland, forming a direct link to the Roman center. Military families were a reality everywhere, with sons often becoming soldiers after their fathers. Thus, the experience of serving in the legions or the auxiliaries tended to promote a shared sense of Roman identity, even when soldiers were drawn from areas that had been conquered by Rome in the recent past.
In the provinces, there was a pattern that took place over a few generations. After being conquered by the Romans, there were often resistance movements and rebellions. Those were put down with overwhelming and brutal force, often worse than that of the initial invasion. Eventually, local elites were integrated in the governor's office and ambitious people made sure their sons learned Latin. Locals started joining the army and, if lucky, returned eventually with money and land to show for it. Roman amenities like aqueducts and baths were built and roads linked the province with the rest of the empire. In short, assimilation happened. A few generations after Roman conquest, many (local elites especially) in a given province would identify with Roman civilization. Regular people in the countryside, meanwhile, would at least be obliged to tolerate Roman rule even if they did not embrace it