13b: The Roman Imperial Military

Augustus's invention of the Principate determined, once and for all, the role of the Roman military. As always, the Roman army was responsible for maintaining the Roman world's internal stability and external defense. But since the Principate was a military tyranny disguised as a republic, control of the army meant control of the government. Augustus needed to assess the Empire's military needs carefully, and maintain just the right size army: not too large or expensive, but strong enough and fast enough to do the job.  Most importantly, the military had to be totally loyal.

Upon consolidating his power over the Roman state, Augustus's first cut the number of Roman troops from roughly 280,000 to roughly 150,000 elite veterans. These veterans swore an oath of allegiance not to Rome, but to Augustus personally. The core of this army was made up of Italian-born Roman citizens, called legionaries, who served a minimum of 25 years. The legionaries were assisted by auxiliary forces recruited from the provinces, also for a minimum term of 25 years. The Romans fought as heavy infantry, but the auxiliaries fought in various roles, depending on their province of origin: different provinces provided different forms of military specialists. Augustus created a third branch, the Praetorian Guard, to serve as his bodyguard. The Praetorians were paid better than the legionaries and were based near Rome. Their commander, the Praetorian Prefect, soon came to play an important role in the Roman government. Tiberius built a barracks for the Praetorians just outside of the city walls, to ensure that he would have access to them, and they to him. As it turned out, the Praetorians had a little too much access to Tiberius when he was murdered by the Praetorian Prefect Macro in 37 AD. Clearly, the Roman army's loyalty had to be earned on a continual basis.

Up through the time of the "Five Good Emperors," the Roman army's loyalty was a near constant. The legionaries were Roman citizens who had volunteered to serve; they recieved land or a sizable bonus after their 25 years were up. Auxiliary soldiers received only one third of a legionary's pay, but became  Roman citizens upon finishing their service. The sons of auxiliaries could and often did serve as legionaries; again, the Roman stock was revitalized by diverse peoples from across the Empire. Granted, the chances of living until retirement were about fifty percent, but the Roman army was still an honored profession and stepping-stone to prosperity. In the field, the Roman legions were usually commanded by capable and reliable ex-consuls personally selected by the Princeps. When the Roman empire was well led, the various legions stayed loyal to the Princeps. Nero's disastrous reign showed that poorly treated legions could and would revolt, but the Year of Four Emperors was an anomaly. The underpinning of the Pax Romana was a strong, well-led, and highly paid military personally controlled by the Princeps himself.

Yet change for the worse was on the horizon. The Roman Empire had reached its natural boundaries under Marcus Aurelius (161-180 AD). The client states destined to become Roman had been added to the Empire. Offensive operations were through once and for all. Yet the Roman Empire's military needs continued to increase, as Rome now had to hang on against the new and fierce "barbarian" tribes appearing on the frontier. The Romans attacked only against raiding tribes or to find a more favorable place to construct fortifications. Defending Rome's border areas, often known collectively as the limes or "boundary," thus became the ultimate concern of all emperors. Sometimes incorrectly understood as a series of walls, the nature of the limes depended on the nature of the area itself. Where natural boundaries, such as rivers like the Euphrates, Rhine, or Danube, or the Sahara desert, were available, the Romans were only too happy to make use of them. Where natural boundaries were not available, the limes contained a series (if not always a line) of fortifications built more to keep watch on the border area than to protect it. Nearby would be a Roman legionary base (castrum) or the base of some auxiliary forces (castellum). The first Roman patrol or watchtower garrison to detect an enemy attack might be defeated, but a suitable force of Romans, usually auxiliaries, could be quickly summoned up to defeat the attackers. The legionaries themselves were not usually needed. But when they did fight- at least up until the Crisis of the Third Century AD - they were all but invincible.

Maintaining the limes required an ever larger military establishment. By Marcus Aurelius's time the Roman army may have numbered as many as 400,000 men, divided roughly half and half between Roman legionaries and foreign auxiliaries.  The majority of soldiers were now recruited from the lower classes in the less Romanized provinces where the legions were stationed. While the new auxiliaries fought well enough, their loyalty towards the Roman Empire was a little questionable. To them, Rome was a faraway city filled with strange people speaking a foreign language. They no doubt participated in the Imperial emperor cult, but they had a much keener sense of their commander's imperium. They no doubt fought under the Roman eagle standards, but their hearts were in their native provinces where they served, not in Rome. If a given army's commander declared himself imperator and made a play for control of the whole Empire, it was often quite happy to fight on his behalf. On some instances, the armies themselves declared their generals as imperator; in these cases, the unlucky general had the choice between leading his troops against the Empire - or death. As we will see, this recurrence of the old "client army" phenomenon would become a recipe for disaster in the third century AD.  

Later on during the Dominate, another system of troops called the limitanei and the comitatenses was set up along the limes. The limitanei were lightly armed troops who lived almost on the border and were intended to take the brunt of invaders' attacks - which became more and more frequent as time went on. The comitatenses were the heavily armed striking force based in nearby cities, analogous to the old Roman legions. Both forces fought hard to protect the borders, but the morale of the limitanei was particularly critical. Like the Roman citizen soldiers of the Republic's glory days, they fought for their lives against dangerous neighbors. But unlike the old citizen soldiers, the limitanei had no stake in the Roman government and no chance of self-advancement through work. Like the phenomenon of the client army, the ultimate dissociation of soldier from civic life would contribute greatly to Rome's troubles.