In the older days of Roman history the fighting forces had been a "citizen army," called out for so long as it was needed, and levied from full and true Roman citizens. In the imperial times with which we are here dealing it had become a standing army. Soldiering was a profession, for which the men volunteered, and, so far as Roman citizens were concerned, it was now seldom, if ever, the case that military service required to be made compulsory on their part. It is true that a young man of the higher classes who proposed to follow a public career, leading to higher and higher offices of state, must have gone through some amount of military training, but no other Roman was actually obliged to serve. The empire was so vast and the total of the standing forces comparatively so small that it was always possible to fill up the legions with those who had some motive or inclination that way. Theoretically the state possessed a claim upon every able-bodied man, but the population of the empire was probably a hundred millions, and to collect a total of some 320,000 soldiers, made up of Roman or romanized "citizens" and of provincial subjects in about equal shares, was a sufficiently easy task, and the recruiters could therefore afford to pick and choose. Above all we must clear our minds of the notion that the Roman soldiers necessarily came from Rome, or even from Italy. They were drawn from the empire at large, and a legion posted in Spain, for example, might be recruited from a special class of Spaniards.

Roughly speaking, the regular army, extending along the frontiers from Chester to Jerusalem and from Jerusalem to Algeria, was composed of two main divisions, called respectively the "legions" and the "auxiliaries." Other special or detached forces - such as the twelve regiments of Imperial Guards and the six of the City Guard - came under neither of these headings, and we may leave them out of the question for the present.

A legion was a brigade of about 6000 infantry, with 120 horsemen attached to it. It was recruited from any convenient part of the empire, but only from men already enjoying the rights of Roman citizens, or else from those other provincials who were considered sufficiently homogeneous with the Roman civilisation to stand shoulder to shoulder with such citizens. In being permitted to serve on these terms a man regularly becomes ipso facto a citizen. The qualifications required were that you should be free-born - that is to say, neither slave nor ex-slave - your physique must be good, and your height about 5 feet 10 inches: there must be nothing serious against your record or character as viewed from the Roman standpoint; and, if you were not already a citizen, you must belong to one of those organised communes which were the units of administration and of taxation within the empire. You undertake to serve for twenty years, after which time you will receive an honourable discharge and either a sum of money - at this date apparently about L50 - or a grant of land. By ability and character you may rise from private soldier to centurion, that is to say, commander of a hundred, but in ordinary circumstances you can climb no further up the military ladder. If at the end of your term you are still robust and are considered useful, you may, if you choose, continue to serve in a special detachment of "veterans," with lighter duties and with exemption from common drill. The Roman legions would thus be made up for the most part of troops from about 18 to 38 years of age, although a considerable number might be somewhat older.

A legion once formed had a perpetual existence; its vacancies were filled up as they occurred; and it is obvious that it must have consisted of respectable men of picked physique, mostly in the prime of life, and perfectly trained in all the qualities of a soldier. When not on actual campaign they were drilled once a day, and the recruits twice. They practised the hurling of spears and all the attitudes of attack with sword and pike, and of defence with the shield. Now and then there was a review or a sham fight. They learned how to fortify a camp, how to attack it or to defend it. Every month they put on full armour, marched out with steady Roman tramp for ten miles and back again to camp for the sake of practice. Meanwhile they were made useful in building the military roads, bridges, and walls. Add to this the strict Roman discipline, and it is difficult to conceive of any training more capable of turning a body of 6000 men into a stubborn and effective fighting machine. The half-naked German across the Rhine was physically as strong and as brave; the woad-dyed Celt of Britain was probably more dashing in his onset; the mounted Parthian across the Euphrates was more nimble in his movements; but neither German nor Celt cultivated the organisation or solidarity of action of the Roman, nor could the Parthian equal him for steady onward pressure or determined stand.