What is the Roman army

Rome was early on (say 500 BCE) a farming community covering about 50 square miles, with a citadel to defend in extremis. The Roman army comprised initially the peasant farmers turning out to repel predatory nomads or neighbours. To be in it you had to possess property. Non-propertied people were not fighting for anything, and so were unreliable; and also it was thought unfair to make them fight to protect others’ property.

As the state became stronger, and population grew, it became necessary to steal the land of neighbours as the one-hectare dry farms were too small to subdivide amongs several sons. So they attacked neighbouring states to get it; the neighbours were also trying to do the same to them. Rome won some and lost some, but on average was more successful at land piracy and spread its control progressively through the Italian Peninsula and across the Padus River to the north into territory settled by the Gauls.

This citizen army (augmented by the Italian allies they had taken under their sway) sufficed until the Germanic peoples got on the move in the late 2nd Century BCE. After a disasterous defeat in Gaul, it became necessary to match the sheer numbers of Germans to recruit from the non-propertied class. This created a new type of solder. Farmers, when a campaign was over, returned to their farms with their share of the loot. These new soldiers had nothing to come back to and so relied on their generals to organise land for them, and this clientele gave the generals the clout to push their own interests, resulting in the civil wars. And of course long campaigns and garrisoning the growing empire turned the citizen warriors into a standing professional army.

After Sulla and Julius Caesar both failed to find a solution to the ambitious generals, Augustus short circuited this by allocating to himself the frontier provinces which is where the legions were and so deprived rivals of a military base. The army was by now completely professional with enlistment terms which grew from 16 to 20 years.

It was still open only to Roman citizens, however it included foreign auxiliariary units for such specialist tasks as archers, slingers, cavalry. And from the early 1st Century BCE the Italian allies were admitted to Roman citizenship, and over the years various other peoples and provinces in Spain, Gaul and Britain were admitted to citizenship. The way was opened finally in 212 CE when Caracalla opened citizenship, and therefore the army, to all free people in the Empire.