Role of Roman Legions in Republic and the Empire

The role of Roman legions in creating the republic first and then the empire has been widely acknowledged. Legions emerged from the early Roman army which was composed of levied citizens and developed into a full fledged force with a modern infantry and well organized cavalry. (McCall, 2002). The first person to conscript soldiers into legions to the advantage of the state was Servius Tullius.
By introducing the census, Tullius made it incumbent for all youth to be organized in various classes based on their income and also made it incumbent for them to join the army as a mark of being Roman citizens. This tradition of use of the legions for political consolidation has thus been the hallmark of the rise of the republic as well as the empire.
In Rome service in the legion was considered mandatory. This naturally led to correlation of the legion with the republic as the concept emerged over the years by consolidation of the state. For the purposes of distinguishing between classes, the recruits were divided into five classes based on their income as each soldier had to acquire his own arm and equipment.

Thus citizenship and legionnaire were both concomitant thereby laying strong linkages between the republic and the legions which were exploited over a period. The legions remained conscripted and were recalled as on required basis during the Republic thereby indicating that these were transient and to be fully exploited for sponsoring the aim of the head of the state.
After the Marian reforms which were brought in at the end of the 2nd century BC, the legion was professionalized by Gaius Marius. This was done by him to enlarge the legions and enable fielding larger armies. Politically too this was important as in a Republic the state had the duty to provide jobs to people, for which legions were considered to be most appropriate organizations, once again highlighting an utilitarian trend.
Assimilation of the Italian soldiers in Roman legions and grant of citizenship was also a Marian initiative thereby enhancing the numbers as well as the power of the state. The ingrained manipulative streak of the masters of the legion would be more than evident in these measures.
This professionalizing also led to a realization that the legions could play an important political role and hence all governors were proscribed from leaving their province with the legions to prevent precipitation of a crisis in other states. Just such a crisis precipitated the civil wars when Caesar broke the rule crossing with his legion into Italy. The civil wars saw the end of the republic and beginning of the Empire led by Augustus in 27 BC.
The legions once again were very effectively used by both Antony and Augustus the ultimate victor of the civil war which led to the establishment of the Roman empire. Once having won the war though Augustus reduced the number of legions as he was finding it difficult to sustain the force. Politically it was not expedient to have many legions which could challenge the authority of the emperor at any given time.
Augustus and then his successors would not however totally abandon the concept, but only added new legions as required by the circumstances and disbanded these when no longer required by the needs of the empire, thereby once again denoting how they successfully exploited the legions for the purposes of the state.
Reference
1.Mccall, Jeremiah B. (2002). The Cavalry of the Roman Republic: Cavalry Combat and Elite Reputations in the Middle and Late Republic. New York : Routledge.