A key distinction in the Roman army was between the professionally trained heavy infantry, known as legionaries, and the support forces, or auxiliaries. The standard strength of a Roman legion was around 5,500 men in the early days of the empire, but became far smaller (1,000 – 2,000) over time. Legions were recruited solely from among the ranks of Roman citizens. They consisted of heavily armed infantrymen whose primary tactical purpose was to lure the main battle line of an enemy into hand-to-hand combat and destroy them. One of the most famous tactics was the testudo or “tortoise” formation. This involved the legionaries closing ranks and grasping their shields at the sides to form a protective shell all around the body of soldiers, including over their heads to make them invulnerable to arrow or javelin attack from above. However, this formation severely limited their manoeuvrability in close quaters combat and so was used primarily as a siege tactic. When an army was in trouble, the formation that it tended to adopt was a square.
Auxiliaries were drawn from the various provinces of the empire or even from client kingdoms. Often these regions contributed forces that specialised in particular fighting techniques. The Gauls and the Batavians, for example, provided cavalrymen, while the kingdoms of Syria and Scythia supplied men who were skilled at archery and the Balearic Islands contributed excellent marksmen with the slingshot. Initially auxiliaries had their own leaders and equipment, though later they became more integrated into the Roman army. Auxiliaries also included more lightly armed infantrymen than the legionaries. In battle, the auxiliaries task was to pin down an enemy force so that the legions could then crush them. This style of fighting, which evolved in the course of the second century BC and was refined by Pompey and Caesar, would remain the basis of Roman tactical doctrine for two more centuries, until Rome's enemies finally devised ways of defeating it.
One thing that emerges very clearly from the works of Tacitus is that he was no blind admirer of every aspect of Roman culture or of Rome's military might. He knew that Roman armies were often poorly drilled and inept and that grave defeats could result from these shortcomings. He also recognised the tendency of Romans to rest on the cultural laurels of the distant past. Finally, he was all too keenly aware that, beyond the empire's northern frontiers, there dwelt many unconquered peoples who might, in time, be impossible to contain. Yet, despite his critical acumen, even Tacitus failed to pinpoint what we, with the benefit of hindsight, can identify as the empire's greatest weakness – the highly traditional nature of Roman thought.
Nowhere is this traditionalism more painfully evident than in the attitude that Romans of the second century AD took to their army. Two short books of this period that treat military matters have come down to us, written by a historian and philosopher of Greek descent called Arrian of Nicomedia. Arrian not only had a practical knowledge of contemporary warfare – as governor of the Roman province of Cappadocia from 130 to 138, he commanded an army that repelled an invasion by the Alan tribes from the steppes of central Asia – he was also a learned scholar of ancient history. In particular, he admired Alexander the Great, and his account of the campaigns of the Macedonian king remains the foundation for modern study of that era. Yet therein lay the problem; for all his undoubted accomplishments on the battlefield, when formulating tactics, Arrian recommends infantry formation that are adaptations of the fourth-century BC phalanx used by his military hero. In writing on cavalry tactics, Arrian is marginally more up-to-date, but even then he commends innovations borrowed from the Gallic and Spanish tribes in the first century BC.
Another surviving text from the same period on fighting tactics comes from North Africa. It is in the form of an inscription recording a series of speeches that Arrian's patron Hadrian gave when visiting the area, praising the drills performed by various auxiliary units. Tellingly, one of these formations is precisely the same Spanish cavalry drill described by Arrian. Apart from providing clear evidence that the two men were well versed in military affairs, the coincidence of this identical tactic being recommended in two different parts of the Roman world also smacks of the dead hand of tradition. The troops that Hadrian was inspecting were the garrison that patrolled the northern boundaries of the sahara, yet there is nothing in his speech to suggest that he took into account the very special requirements of desert warfare.
Certainly, there was no centralised drill manual, and Roman armies had been known since the days of the republic for adopting the fighting and campaigning skills of their adversaries. For example, the ploy of troops using inflated animal skins to swim across rivers had been learned from the Batavian tribes of northern Belgium. However, these days lay far in the past, and for a long time no new strategic or tactical thinking had been evolved to cope with changed circumstances.
This same complacent attitude was still in evidence during the reign of Marcus Aurelius. In a manual on generalship entitled The Stratagem of War, a Macedonian writer named Polyaenus gathered together the successful stratagems of famous commanders of the past. He dedicated his book to Marcus and Verus, who at the time were campaigning against the Parthians. The sweeping, anecdotal approach taken in Polyaenus' book to the art of war stood in a long and honourable tradition of learning from the past. Nor were such writings the only source that an aspiring Roman general could consult; at the other end of the spectrum stood the kind of detailed treatise on practical aspects of warfare exemplified by Apollodorus of Damascus' work on siege engines. In this, Trajan's chief military engineer during the Dacian Wars urge his successor Hadrian to heed the vital importance of technical data in the construction of such machines.
Yet, for all their ostensible differences, these two works both focus on minutiae of tactics in preference to analysing more fundamental strategic problems. Senior Roman officers were trained, first and foremost, in the arts of administration. And this, in a nutshell, was the Achilles' heel of the Roman army in the second century AD. Rome had no war college, and Roman commanders' understanding of warfare seems primarily to have been based upon the analysis of examples of past success rather than a wider appreciation of the factors that underlay that success. The most renowned generals of history – Alexander, Hannibal and Caesar – had all had the ability to adjust rapidly to changing circumstanced. The generals who served under Marcus Aurelius, though perfectly competent, seem rather to have favoured tradition over experimentation. Ironically, in learning by rote the career details of their illustrious predecessors, they failed to take on board precisely those essential, spontaneous qualities – innovation and flexibility – that made them truly great. And so, notwithstanding its initial success, Marcus Aurelius' invasion of Persia in AD 163 was based on a plan devised by Julius Caesar in 44 BC, while the northern wars fought during his reign dragged on because the enemy steadfastly refused to offer battle under conditions that Roman commanders had been schooled to deal with.
The issue that Tacitus highlighted – cultural complacency and the vast expanse of the non-Roman world to the north – would come to dominate the 80 years following Marcus Aurelius' death. Beginning with internal upheaval, the empire's problems were later compounded by the suddern emergence of a militarily competent Persian dynasty in the 270s. Persian success would so weaken Rome's central government that its defences against the northern tribes crumbled. Consequently, for more than a decade after 260, the empire split into three parts. The catastrophes of the 250s and 260s ushered in an era of radical reform rivalling that which saw the emergence of the imperial office in the time of Augustus. By 337, the Roman emperor was a Christian and Rome had ceased to be the centre of imperial government.
THE NEW ROMAN ARMY
The armies that Carinus and Diocletian led against one another in 285 were radically different from the legionary forces that had been the mainstay of Rome's army until the mid-third century. The old Roman army had experienced its demise in the military catastrophes that occurred under Decius, Gallus and Valerian. Although there were still many units called legions, these were no longer the large bodies comprising some 5,500 heavily armed and armoured infantrymen that could trace their lineage back to the armies of the Republic. Legions now tended to number no more than 2,000 men, were more lightly armoured and trained in new tactics. The main strike force of the army, which was gradually being separated from units that were primarily assigned to frontier protection, contained higher proportions of cavalry than was the norm before 250. In many ways it was a better army, capable of a wider range of tactical manoeuvres, and capable of adapting to a variety of enemies. Unlike the army of the Severan era, the new Roman army was designed to fight Rome's enemies on their own terms. The new look army enjoyed notable success from the opening year of Aurelian's reign onwards.