Roman Republic
Late Republican legions

The organisation of the legions evolved throughout the Republican period. In 107, all citizens, regardless of their wealth or social class, were made eligible for entry into the Roman army. The distinction among the three heavy infantry classes, which had already blurred, had collapsed into a single class of heavy legionary infantry. The heavy infantry legionaries were drawn from citizen stock, while non-citizens came to dominate the ranks of the light infantry. The army's higher-level officers and commanders were still drawn exclusively from the Roman aristocracy. Unlike earlier in the Republic, legionaries were no longer fighting on a seasonal basis to protect their land. Instead, they received standard pay and were employed by the state on a fixed-term basis. As a consequence, military duty began to appeal most to the poorest sections of society, to whom a salaried pay was attractive.

The legions of the late Republic were almost entirely heavy infantry. The main legionary sub-unit was a cohort of approximately 480 infantrymen, further divided into six centuries of 80 men each. Each century comprised 10 "tent groups" of eight men. Cavalry were used as scouts and dispatch riders rather than as battlefield forces. Legions also contained a dedicated group of artillery crew of perhaps 60 men. Each legion was normally partnered with an approximately equal number of allied (non-Roman) troops.

The army's most obvious deficiency lay in its shortage of cavalry, especially heavy cavalry. Particularly in the East, Rome's slow-moving infantry legions were often confronted by fast-moving cavalry troops and found themselves at a tactical disadvantage.

After Rome's subjugation of the Mediterranean, its navy declined in size, although it underwent short-term upgrading and revitalisation in the late Republic to meet several new demands. Julius Caesar assembled a fleet to cross the English Channel and invade Britannia. Pompey raised a fleet to deal with the Cilician pirates who threatened Rome's Mediterranean trading routes. During the civil war that followed, as many as 1,000 ships were either constructed or pressed into service from Greek cities.
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