Nowhere does the Roman talent for organization show itself so clearly as in its army. The story of the Roman army is an extensive one, demonstrated in part by the scale of this chapter.
The first part of this chapter considers the history of the Roman army (concentrating on the legions), trying to explain as much background as possible. The later part of the chapter seeks to explain specific points such as various different units, the workings of the army, etc. 

The Greek Phalanx

The early Roman army, however, was a different thing altogether than the later imperial army.
At first, under the Etruscan Kings, the massive Greek phalanx was the mode of battle. Early Roman soldiers hence must have looked much like Greek hoplites.
A key moment in Roman history was the introduction of the census (the counting of the people) under Servius Tullius. With this the citizens were graded into five classes, from these classes were in varying degrees recruited the ranks of the army. The most wealthy, the first class, were the most heavily armed, equipped like the Greek hoplite warrior with helmet, round shield, greaves and breastplate, all of bronze, and carrying a spear and sword.
The lesser classes bore lesser armament and weaponry, the fifth class carrying no armour at all, solely armed with slings.
The army officers as well as the cavalry were drawn from leading citizens who were enrolled as equestrians (equites).

All in all the Roman army consisted of 18 centuries of equites, 82 centuries of the first class (of which 2 centuries were engineers), 20 centuries each of the second, third and fourth classes and 32 centuries of the fifth class (of which 2 centuries were trumpeters).

In the early fourth century BC Rome received its greatest humiliation, as the Gauls sacked Rome itself.
If Rome was to reestablish her authority of central Italy, and be prepared to meet any similar disasters in future, some reorganization was needed. These changes were traditionally by the later Romans believed to have been the work of the great hero Fluvius Camillus, but it appears more likely that the reforms were introduced gradually during the second half of the fourth century BC.
Undoubtedly the most important change was the abandonment of the use of the Greek phalanx.
Italy was not governed by city states like Greece, where armies met on large plains, deemed suitable by both sides, to reach a decision. Far more it was a collection of hill tribes using the difficult terrain to their advantage. Something altogether more flexible was needed to combat such foes than the unwieldy, slow-moving phalanx.