The Roman Army was the backbone of the Kingdom, the Republic and the Empire. It was a citizen army, with annual conscription of citizens as levy. The army shaped the boundaries of Rome and preserved its memory by preventing annihilation by foreign forces and those within. Needless to say, the army won the Romans their empire.
Hoplite Armies (500 BC – 315 BC)
The Early Roman Army was chiefly responsible for small-scale raids, modeled on the Etruscan or Greek Armies. The Roman Army was based on an annual levy (. Legion was derived from legio that is Latin for levy). Since every man had to provide his own equipment, the wealthiest, who could afford the best weaponry, at the front rank. Each ensuing rank comprised of those with less wealth than the one before it. The poorer strata formed the infantry and the rich formed the equites or cavalry. The phalanx formation of the army, that included 3000 infantry and 300 cavalry, was in effect till the Roman military catastrophe of 390 BC at the Battle of Allia, after which it was replaced by the more flexible manipular formation.
Manipular Army (315 BC –107 BC)
The army of mid-Republic Rome was also called the Manipular Army or the Polybian Army(named after Polybius, the historian who gave the most detailed account of these armies). The army started to maintain a permanent strength of 150,000 soldiers while the remaining three quarters were levied. During this period, armies were organized into legions of around 5,000 men (of both heavy and light infantry). The manipular army was based on social class, age and military experience.
Maniples were units of 120 men each drawn from an infantry class. They were positioned as three lines based on the three heavy infantry types. The first line maniple were the hastati, unproven men who wore a bronze breastplate and a bronze helmet, armed with a sword and two throwing spears. The second line maniple were the principes. Older men with some military experience were armed and armored in the same manner as the hastati, but wore a lighter coat of mail. The triarii formed the third line. They were the last remnant of the hoplite-style troops in the Roman army. Veteran troops of advanced age and experience were armed and armored like the principes, but carried a lighter spear. The heavy infantry of the maniples were supported by a number of light infantry and cavalry troops, typically 300 horsemen per legion.
During this time, Rome bound all the other states of the Italian peninsula into a permanent military confederation that meant that they supplied half of Rome’s army. The Socii (the Etruscans, Umbrians, Apulians, Campanians, Samnites, Lucani, Bruttii, and the various southern Greek cities) were organized in alae, or wings.
After the Second Punic War, Roman armies started hiring non-Italian mercenaries including Numidian light cavalry, Cretan archers and Balearic slingers.
Legion After the Reforms of Gaius Marius (107 BC – 27 BC)
Maniples were eventually phased out, and replaced by larger cohorts (approximately 480 infantrymen, further divided into six centuries or 10 “tent groups” of 8 men) as the main tactical unit of the Roman Army. Roman consul Gaius Marius, through the Marian reforms, changed the face, structure and organization of the Roman Army. In 107 BC, all citizens, regardless of their wealth or social class, were made eligible for entry into the Roman army. The heavy infantry legionaries were drawn from citizens, while non-citizens controlled the ranks of the light infantry. The legions of the late Republic were almost entirely heavy infantry.