century, all
able-bodied men holding property valued at 4000 asses were regularly
called upon for military service. The others were liable to naval service,
but only in cases of great need were they enrolled in the legions.
Ordinarily, the service required amounted to sixteen campaigns in the
infantry and ten in the cavalry. The field army was raised from those
between seventeen and forty-six years of age: those forty-six and over
were liable only for garrison duty in the city. The regular annual levy
consisted of four legions, besides 1800 cavalry. This number could be
increased at need, and the Roman forces in the field were supplemented by
at least an equal number in the contingents from the Italian allies.

The Roman army was thus a national levy: a militia. It was commanded by
the consuls, the annually elected presidents of the state. Yet it avoided
the characteristic weaknesses of militia troops, for the frequency of the
Roman wars and the length of the period of liability for service assured
the presence of a large quota of veterans in each levy and maintained a
high standard of military efficiency. Furthermore, the consuls, if not
always good generals, were generally experienced soldiers, for a record of
ten campaigns was required of the candidate for public office. Likewise
their subordinates, the military tribunes, were veterans, having seen some
five and others ten years' service. But the factor that contributed above
all else to the success of the Roman armies was their iron discipline. The
consular _imperium_ gave its holder absolute power over the lives of the
soldiers in the field, and death was the penalty for neglect of duty,
disobedience, or cowardice. The most striking proof of the discipline of
the Roman armies is that after every march they were required to construct
a fortified camp, laid out according to fixed rules and protected by a
ditch, a wall of earth, and a palisade for which they carried the stakes.
No matter how strenuous their labors had been, they never neglected this
task, in striking contrast to the Greek citizen armies which could not be
induced to construct works of this kind. The fortified camp rendered the
Romans safe from surprise attacks, allowed them to choose their own time
for joining battle, and gave them a secure refuge after a defeat. It
played a very large part in the operations of the Roman armies, especially
such as were conducted in hostile territory.