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.
The Early Legion (4th century BC)
In abandoning the phalanx, the Romans showed their genius for adaptability.
Though much of the credit might not be due to the Romans alone. For Rome was a founding member of the Latin League, an alliance initially formed against the Etruscans. The development of the early legion therefore might well be seen as a Latin development.
There were now three lines of soldiers, the hastati in the front, the principes forming the second row, and the triarii, rorarii and accensi in the rear.
At the front stood the hastati, who were most likely the spearmen of the second class in the previous organization of the phalanx. The hastati contained the young fighters and carried body armour and a rectangular shield, the scutum, which should remain the distinctive equipment of the legionary throughout Roman history. As weapons they carried a sword each and javalins. Though attached to the hastati were far more lightly armed skirmishers (leves), carrying a spear and several javelins.
The soldiers of the old first class now appear to have become two types of units, the principes in the second line and the triarii in the third line. Together they formed the heavy infantry.
The principes were the picked men of experience and maturity. They were similarly, though better equipped than the hastati. In fact the principes were the best equipped men in the early legion.
The triarii were veterans and still much looked and functioned like the heavily armed hoplites of the old Greek phalanx.
The other new units, the rorarii, accensi (and leves) represented what once had been the third, fourth and fifth class in the old phalanx system.
The rorariiwere younger, inexperienced men, and the accensi were the least dependable fighters