To most people, this is the 'classic' era of  the Roman army, when legionaries equipped  with the famous lorica segmentata used javelin and short sword to subdue the barbarians and bring the Pax Romana to the known world. These were tough, well trained, superbly equipped and disciplined soldiers who collectively constituted the most formidable fighting machine that the Ancient World had ever seen. Nor is this popular view far removed from the truth.

The Achievements of the Divine Augustus.   
It was the first Emperor of the Roman world, Augustus (-AD14) who transformed the politically divisive and somewhat ad hoc military forces of the Republic into a fully professional army with a consistent policy towards the security of the Empire.

However much he may have tried to hide the fact from an adoring public, Augustus was never under any illusions as to the military basis of his authority, making very sure that the first and only loyalty of Rome's troops was to him alone and, through him, to the Roman state. It was Augustus who made sure that they were paid regularly ( three times a year at this time), generally out of his own resources, it was Augustus who organised their retirement handouts of cash or land, and it was to Augustus that the troops swore their oaths of allegiance every January 1^st. The Republic had failed to do any of these essential tasks and had paid the ultimate price for its neglect of the military

Equally, it was Augustus who regularised their length of service, appointed all military commanders and organised the campaigns (though seldom in person) which brought them glory, booty and popular acclaim as the conquerors of more territory than at any other time in the history of the Roman state. It need not surprise us that they were devoted to Augustus and his family; indeed, the first Emperor did his job so well that the army remained conspicuously loyal to any Emperor who possessed the Julian name, even 'difficult' ones like Caligula and Nero.

Indeed, for most of the first two centuries of Roman rule the army, with the possible exception of the Praetorian guard, seemed to have abandoned its political pretensions, though it was a foolish princeps who did not pay close attention to his soldiers' welfare.

Augustus actually inherited some 60 legions after the battle of Actium (31BC ) but soon reduced this to 28 (c.150,000men) in the interests of political security and his bank balance, ensuring that they were carefully distributed throughout the Roman world in accordance with military imperatives and his own political security- too many legions in one place might tempt unscrupulous governors to emulate his rise to power.
The main concentrations were on the Rhine (8 legions), the Danube (7 legions)  and Syria  (4  Legions),  though Egypt, Africa and Spain also possessed permanent legionary garrisons.

Of course, the Roman army was not simply comprised of citizen heavy infantry - other troops were vital to perform those tasks that disdainful legionaries would not countenance, such as providing cavalry, or firing arrows or scouting and it was again Augustus who created a new professional force, the auxilia (literally 'help troops') out of the irregular units of allies who had helped Rome conquer most of her Empire in Republican times.

These auxiliaries also numbered over 150,000 men in total,  organised into units 500 or, less frequently, 1,000 strong, and were raised from the non-citizen inhabitants of the provinces and newly conquered lands. With their keen fighting edge and specialised weapons and tactics, they soon became an indispensable part of the Roman army, generally going into battle first, and sometimes showing themselves  capable of winning significant victories without  involving the legions at all.

Legionary organisation - citizen troops -a summary

The commander of a legion in our period was invariably a Senator of some standing - a man who had already had a varied military and administrative career, and who had probably reached the rank of  praetor at Rome. His official title was legatus legionis because, while theoretically the Emperor held all military power, in  practice he delegated it to trusted Senators. Such legionary legates were, on the whole, tough, experienced and highly competent officers,  though there were some notable exceptions, and all must, at some time, have served as junior officers or tribuni.

These young  tribunes were known as tribuni ticlavii -'broad stripe men' ( after the broad purple band found on the Senatorial toga) doing a short stint as part of the Senatorial career structure and acting as staff officers and aides-de-camp to the legate. More useful, however, were the tribuni angusticlavii , the 'narrow stripe men' of Equestrian rank., professional soldiers who had made the army their career and who could be expected to know their business somewhat better.

Undoubtedly the most important officers in the legion, however, were the centurions of which there were 60, one to each century, tough, seasoned soldiers who were generally promoted from the ranks  and whose quality was assured by the fierce competition which existed, first to enter the centurionate and then to gain promotion through the various centurial grades to the exalted rank of Chief Centurion or Primus Pilus.
Centurions were distinguished by their equipment, ( including a vitis or vine staff used to beat recalcitrant legionaries), their more spacious barrack room accommodation, their relatively high levels of pay and their privileges which included taking bribes to exempt soldiers from unpleasant duties. The downside of life as a centurion was that he was expected to lead his men from the front, so  presumably casualties in battle must have been disproportionately high amongst this particular rank.

The Principales

N.C.O's  within the century included the Signifer who bore the signum or standard of the century, (which was, of course,  the embodiment of the honour and spirit of the unit) and who also acted as its banker; the Optio, so called because he had been chosen by the centurion with a view to promotion tothecenturionate himself; and the Tesserarius who was in charge of passing on the watchword.of the day. Other NCO's were to be found at legionary headquarters dealing with the vast amount of paperwork generated by this highly bureaucratic army.

Milites Gregarii
The majority of soldiers were, of course, ordinary footslogging milites, although even these had gradations, with soldiers who had earned exemption from unpleasant duties being known as immmunes and those who had served their time gaining the status of veteranus along with appropriate privileges.