Warmatrix

The Roman empire lasted many centuries and during that time its army changed its organization, recruitment, equipment and tactics many times. Around the 3rd century BCE the first truly Roman army took shape, after reforms of Marcus Furius Camillus and Rome's wars with the Samnites.
In the early centuries of its existence, the Roman army was but a shadow of its later greatness. Roman soldiers were militiamen, mostly unarmored and fought only small skirmishes with neighboring cities: cattle raiding and such. When the city grew, it established a militia army along Greek lines: a phalanx armed with spears and bronze armor, supplemented with light troops and cavalry. The basic tactical unit was the "centuria", consisting of about 100 men, as the name implies. Around 400 BCE limited pay was introduced, enough to cover food, clothing and equipment. Still, the army was a militia force, operating only part of the year and returning home when the campaigning season was over.
In the early 3rd century BCE, after their wars with the Samnites of southern Italy, the Romans changed their battle line. Instead of a single mass of soldiers, they adopted a chessboard-like formation of three lines, later called the "quincunx". In it, small units of 120 men, the "manipels", could operate much more freely. The Romans probably copied the formation from the Samnites, who had learned how to fight in the rugged terrain of south Italy. Young soldiers in the first line were called "hastati", older ones in second line "principes". The name hastati is an echo of earlier times, when each man wielded a "hasta" (spear), but the new hastati and principes fought with short swords. During the battle, they alternated, so that one line could rest while the other fought. When things got dire, the third line moved up. This consisted of veterans, the "triarii", who still wielded spears.
The infantry manipels were grouped into "cohorts" of 4 manipels (480 men), which were administrative units. Cavalry units were called "turmae" and held 30 men. The strategic unit of the Romans was the legion, normally 3,000 heavy infantry, 1,000 velites and 300 cavalry, for a total of 4,300 men, though the actual number varied between 4,000 and 5,000. The manipels were commanded by two "centurions", a senior and a junior. The middle ranks of of the army were made up of "tribunes", elected by the people, and the top by "consuls" and "praetors", elected by the senate.
The infantry was recruited from property-owning Roman citizens; the poor, old people and slaves were excluded. The soldiers had to pay for their own equipment, though they received a salary. Next to the heavy infantry there were the "velites", mostly young men with little armor, armed with javelins only and used as skirmishers. The cavalry, the "equites", were recruited from the wealthier citizens, who could afford to pay for the horses.
Around the Punic Wars the tactical reorganization was followed up by a change and standardization of equipment. The infantry were equipped with helmets, either a "lorica squamata" (scale armor) or "lorica hamata" (mail armor) and a large shield, the "scutum". When they closed with the enemy, they opened the fight by throwing two javelins, called "pila". At close distance a Roman infantryman used his shield to knock the other side off balance and then tried to deliver a belly stab with his short sword, the gladius, of Iberian origin. The shock of a clash with Roman infantry and the horrible wounds that their swords inflicted, terrified most opponents, who were used to longer fights and lighter spear wounds.
On the march, Roman armies had a habit of constructing a fortified camp every night. These took time and effort to construct, but guaranteed sound sleep in the safety of a small fortress. This was a typical Roman feature that shows how much they were inclined to engineering, both in peace and in war.
The revised Roman republican army was a semi-professional but still essentially militia force, levied at the start of a campaign and disbanded afterwards. The core was recruited from Roman citizens, their number increased by their allies. The latter legions were called "alae", 'wings', because they were often deployed on the flanks. They held smaller numbers of infantry and greater of cavalry and were led by Roman commanders. The allies had to supply soldiers and delegate control of the war to Rome, but there were substantial advantages too. Rome supplied twice as many soldiers as the allies, the combined strength deterred many enemies and booty was shared on a 50-50 basis. The rewards of military service and the vast pool of manpower explain why the Romans, even when confronted with defeats like those against Pyrrhus of Epirus and Hannibal Barca, managed to hold their realm together.