Republic
Under the Republic, the legions are made up of citizen soldiers, who leave their ordinary activities to defend the city (and their own property). To carry out the raising of the legions, all the Roman citizens met, in the spring, on the Champ de Mars.
The citizens are divided into 193 groups according to their wealth, established by the censor during the censuses which took place every five years (lustre). The first 98 census classes are considered well-to-do and have low numbers. The following are made up of peasants owning their land, and more or less prosperous.
The choice of citizens to constitute the legions is entrusted to the gods, by drawing lots. Each of the censitaires classes mentioned above must provide enough men to constitute a centurie. The classes were therefore called centuries. We therefore have, in the numerically small centuries (censitaires) of rich citizens, a high proportion which is drawn by lot, to constitute a complete (military) centuria. Conversely, in the numerically significant poor centuries (groups of citizens), a small proportion of the centuria (group of citizens) is chosen by lot. This system is justified in two ways:
* first, because citizens fight to defend their property, the rich obviously have more to defend than the poor, and therefore it is considered normal that they defend it themselves;
* then, as the citizens paid for their equipment themselves, it is easier for a wealthy man to cover this expense. We therefore have better equipped legionnaires, and therefore a more valiant legion.
When drawing lots, each man is called by name. He then steps out of the ranks, indicates whether or not he can serve the legion this year, gives his excuse which is examined immediately, and is accepted or not. The draw continues until the legions are complete. If more legionnaires are needed, several military centuries are drawn by lot per civic century, starting with the equestrian centuries and ending with the proletarian century (which can provide one century less than the others).
The first eighteen centuries provide the cavalry. The citizens composing them are the only ones who can provide their horse. These centuries are called equestrian for this reason.
Under the Republic, in ordinary times, 4 legions are raised each year. During the Second Punic War, the numbers were 6 legions in 218 BC. J.-C. at the beginning of the war and reached 23 legions in 211.
At the end of the Republic and under the Empire, from Augustus to Diocletian
At the end of the 2nd century BC. J. - C., the military campaigns being longer and more distant, the consuls raise troops among the poor peasants and pay them a balance. This is the end of the army of citizen soldiers.
The Roman army evolved little until the middle of the 3rd century, sticking to the achievements made by Augustus and then by Hadrian. At the height of the empire, 350,000 men were enough to cover a border of almost 10,000 km.
This workforce, divided into around thirty legions and auxiliary corps, had to work to reduce one or even two enemy forces in a sometimes restricted area. But these tactical designs were increasingly unresponsive to the extreme mobility of the new enemies.
In the 3rd century, such a force was no longer sufficient to deal with the multiplicity of conflicts which sometimes broke out simultaneously on all the borders of the empire.
An army composed essentially of infantrymen, flanked of a reduced cavalry, remained powerless against a mobile enemy, fleeing, practicing guerrilla warfare and refusing pitched battle on open ground for as long as possible. The legion of 4,500 to 6,000 men, as it could still appear at that date, was reaching its limits.
What made it strong became its main handicap. Too heavy, too slow, the enormous logistics that a legion and its auxiliaries involved slowed down its operations. Once the defensive curtain (limes) was forced, nothing could stop the border barbarian groups in their plundering enterprises. The surprise, the sudden change of direction making them unpredictable, entire cities fell into the hands of the invader without even using poliorcetics! Some barbarian peoples did not neglect to observe their Roman adversaries, and ended up practicing the same combat techniques. All of these factors contributed in part to making the ancient legion obsolete. Added to this was the economic crisis, inflation, slow communications and lack of coordination in times of civil war and invasion. The need for a profound reform of the army became indispensable.