The Roman legion was a most perfect organization, a great mechanical force, and could sustain furious attacks after vigor, patriotism, and public spirit had fled.  For three hundred years a vast empire was sustained by mechanism alone.  The legion is coeval with the foundation of Rome, but the number of the troops of which it was composed varied at different periods.  It rarely exceeded six thousand men; Gibbon estimates the number at six thousand eight hundred and twenty-six men.  For many centuries it was composed exclusively of Roman citizens.  Up to the year B.C. 107, no one was permitted to serve among the regular troops except those who were regarded as possessing a strong personal interest in the stability of the republic.  Marius admitted all orders of citizens; and after the close of the Social War, B.C. 87, the whole free population of Italy was allowed to serve in the regular army.  Claudius incorporated with the legion the vanquished Goths, and after him the barbarians filled up the ranks on account of the degeneracy of the times.  But during the period when the Romans were conquering the world every citizen was trained to arms, like the Germans of the present day, and was liable to be called upon to serve in the armies.  In the early age of the republic the legion was disbanded as soon as the special service was performed, and was in all essential respects a militia.  For three centuries we have no record of a Roman army wintering in the field; but when Southern Italy became the seat of war, and especially when Rome was menaced by foreign enemies, and still more when a protracted foreign service became inevitable, the same soldiers remained in activity for several years.  Gradually the distinction between the soldier and the civilian was entirely obliterated.  The distant wars of the republic—­such as the prolonged operations of Caesar in Gaul, and the civil contests—­made a standing army a necessity.  During the civil wars between Caesar and Pompey the legions were forty in number; under Augustus, but twenty-five.  Alexander Severus increased them to thirty-two.  This was the standing force of the empire,—­from one hundred and fifty thousand to two hundred and forty thousand men, stationed in the various provinces.

The main dependence of the legion was on the infantry, which wore heavy armor consisting of helmet, breastplate, greaves on the right leg, and on the left arm a buckler, four feet in length and two and a half in width.  The helmet was originally made of leather or untanned skin, strengthened and adorned by bronze or gold, and surmounted by a crest which was often of horse-hair, and so made as to give an imposing look.  The crests served not only for ornament, but to distinguish the different centurions.  The breastplate, or cuirass, was generally made of metal, and sometimes was highly ornamented.  Chain-mail was also used.  The greaves were of bronze or brass, with a lining of leather or felt, and reached above the knees.