The Roman army from the beginning of its existence was constituted by Roman citizens who had property/land allowing them to arm themselves in battle. The change occurred at the end of the 2nd century BCE, when Gaius Marius first extended his hand to people without land (proletarii), offering them military service and a form of existence.
This was due to problems with consumption. Few recruits came to the army who did not see much sense in the fight, risking their lives or losing income from the farm. In the face of enormous threat from Cimbri and Teutons (109 – 101 BCE), Mariusz decided to volunteer, thus being able to stand up to the barbarian masses. This practice was later followed by subsequent leaders. People volunteered to legions, knowing that nothing but hunger and poverty was waiting for them in the cities. What’s more, they counted on the profits of the campaign and the possible distribution of loot, and even land grants.
In history, the belief that Gaius Mariusz transformed the Roman army from universal to professional was established. This is not true. Mariusz, like other Roman commanders of that period (e.g. Scipio the Younger) had to deal with recruitment problems. However, he did not completely abandon universal conscription and continued to recruit citizens with appropriate property census.
Citizens who accompanied the leaders in longer campaigns began to treat the “warrior” as a form of profession and gained the status of veterans. Moreover, the Republic needed a regular army that would be able to be stationed in remote provinces (e.g. Spain). This is the opinion of R. E. Smith, who additionally suspected that the Roman army of the 1st century BC began to divide into legions:
- permanent – stationed at the borders of the state; composed largely of volunteers (often with long experience) who saw a way of life in the army. Importantly, the armament gradually unified; the differences between the censuses have ceased to be seen.
- temporary – organized in the event of war and short war operations; largely constituted by conscription and by young people.
With the victory in the civil war in 31 BCE, Octavian Augustus faced the decision on how to ensure the loyalty of all legions and avoid the future of a rebellious, ambitious leader who will sue the loyalty of new troops. The examples of Mariusz, Sulla, and Caesar – the leaders who gained the trust of their legions – prove that the threat from potential opponents was still real.
For this reason, Octavian Augustus decided to leave the mixed army: made up of professionals and citizens from conscription. Every year, soldiers took the oath (sacramentum militare) for loyalty to Rome and the consul. Due to the fact that Augustus officially sat in the senate between the two consuls, possessed consular insignia (fasces) and possessed the so-called imperium consulare maius, loyalty was to him.
Augustus decided to establish a fully professional army, to which anyone with Roman citizenship could recruit. M. Cary and H. H. Scullard argue that it is very likely that in the absence of Roman citizens or their insufficient number in the region concerned, it was agreed to incorporate the legions of the local community. This type of approach gradually increased in the second century CE. As an example, Britain was mentioned, which in the first century CE it had only about 50,000 Roman citizens with almost a population of two million in areas controlled by the Romans.
In order to encourage citizens to join the army, Augustus even decided to guarantee a one-time bonus of 3,000 denarii (in 5 CE). A special military treasury (aerarium militare) was created to cover military expenses; moreover, Augustus imposed a 5% inheritance tax and a 1% auction/sale tax. The most common practice was still giving land to veterans after service, which was especially popular in the colonies. The legionary could count on a pay of 225 denarii per year.