The barbarians began to beat upon the walls of the empire as early as AD 160....they came on horseback, bringing new tactics for the Roman infantry to face, and they came in masses. We may doubt if any military sustem could have permanently stayed this series of human tides. But the Empire did what it could....

From the Brittanica

The Roman Army

In the Roman army, the commanding officer of a legion was called the Legate. He was assisted by a deputy called the Camp Prefect, and a staff of six senior administrative officers called Tribunes. The original function of the Tribunes was to spread the call to arms and to ensure that the citizens rallied to the Eagles in time to march and fight. Later, the Tribunate became more of a political tenure, a training ground for young noblemen waiting to go into the consular or civil services. Whenever a Tribune chose to distinguish himself militarily rather than serve his time administratively and get out, his success was almost preordained.

There were normally 28 legions in commission at any given time, and each legion was divided into 10 cohorts. By the end of the third century, the first two cohorts of each legion had been expanded to Millarian status, which meant that each held 1,000 men and was the approximate equivalent of the modern battalion. Prior to that time, only the First Cohort had been Millarian. To the First and Second Cohorts fell the honor of holding the right of the legion's line of battle, and they were made up of the finest and strongest battle-hardened veterans. Cohorts Three through Ten were standard cohorts of 500 to 600 men. Each Millarian cohort was composed of ten maniples, and a maniple was made up of ten squads of 10 to 12 men each.

The bulk of the legion's command was provided by the Centuriate, from the ranks of which came the centurions, all the middle-and lower-ranking commissioned officers of the legion. There were six centurions to each cohort from Three to Ten, making 48, and five senior centurions called primi ordines, in each of the two Millarian Cohorts. Each legion had a primus pilus, the senior centurion, a kind of super- charged Regimental Sergeant Major. The primus pilus headed the First Cohort, the Second Cohort was headed by the princeps secundus, and Cohorts Three through Ten were each commanded by a pilus prior. The Roman centurion was distinguished by his uniform: his armor was silvered, he wore his sword on his left side rather than his right, and the crest of his helmet was turned so that it went sideways across his helmet like a halo.

Each centurion had the right, or the option, to appoint a second-in-command for himself, and these men, the equivalents of non-commissioned officers, were known for that reason as optios. Other junior officers were the standard bearers, one of whom, the aquilifer bore the Eagle of the legion. There was also a signifier for each century, who bore the unit's identity crest and acted as its banker.

Each legion also had a full complement of physicians and surgeons, veterinarians, quartermasters and clerks, trumpeters, guard commanders, intelligence officers, torturers and executioners.

The Roman Cavalry

By the end of the second century AD, cavalry was playing an important role in legionary tactics and represented up to one-fifth of overall forces in many military actions. Nevertheless, until the turn of the fifth century, the cavalry was the army's weakest link. The Romans themselves were never great horsemen, and Roman cavalry was seldom truly Roman. They preferred to leave the cavalry to their allies and subject nations, so that history tells us of the magnificent German mixed cavalry that Julius Caesar admired, and which gave raise to cohortes equitates, the mixed cohorts of cavalry and infantry used in the first, second and third centuries AD. Roman writers also mention with admiration the light horsemen of North Africa, who rode without bridles.

Fundamentally, with very few exceptions, cavalry was used as light skirmishing troops, mainly mounted archers whose job was patrol, reconnaissance and the provision of a mobile defensive screen while the legion was massing in battle array. Roman cavalry of the early and middle Empire was organized in alae, units of 500 to 1,000 men divided into squadrons, or turmae, of 30 or 40 horsemen under the command of decurions. We know that the Romans used a kind of saddle, with four saddle horns for anchoring baggage, but they had no knowledge of stirrups, although they did use spurs. They also used horseshoes and snaffle bits, and some of their horses wore armored cataphractus blankets of bronze scales, although there is little evidence that this form of armor, or armored cavalry, was ever widely used.

Until the fifth century, and the aftermath of the Battle of Adrianople, it would seem that almost no attempt had been made to study the heavy cavalry techniques used in the second century BC by Philip of Macedon and his son Alexander the Great. It was that renaissance, allied with the arrival of stirrups in Europe somewhere in the first half of the fifth century, that changed warfare forever. In terms of military impact, the significance of the saddle with stirrups was probably greater that the invention of tank.