For most of the history of the Roman Empire, the legions formed the backbone of the Empire's military and political power. However, the familiar figure of the legionnaire represents only one aspect of the Roman army. Numerically, the bulk of the Roman army consisted of auxiliary soldiers drawn from Rome's provincial holdings.
Who Were the Auxiliaries?
The auxiliaries were soldiers from provinces which were ruled by the Roman Empire, but who were not born into Roman citizenship. Auxiliaries were granted citizenship after they had served in the army for 25 years. Since citizenship was hereditary, this was a powerful incentive. As a result, auxiliary soldiers were volunteers, not conscripts, and formed a loyal and disciplined part of the Roman legion. They filled many combat roles that the heavy infantry of the legionaries were ill-suited for.
The infantry units of the auxiliaries shared the same organizational structure of the legionaries, but were armed with different equipment. Their primary role was as light infantry. By wearing lighter armour and often going without shields, these light infantry units were able to outmanoeuvre and pursue enemy units who might have otherwise flanked the heavy legions. These auxiliaries wielded spears, long swords or axes, largely depending on the military traditions of their home province.
The auxiliary cavalry units were vital to the success of the Roman army. The Roman legion did not have a history of strong horsemanship, and the vast majority of the cavalry units in the Roman army were auxiliaries recruited from provinces with historic ties to horseback riding. These units ranged from elite heavy lancers to light scouts and skirmishers, and even included horse archers. The auxiliary cavalry units added an indispensable range of troops to the Roman army.
Many of the auxiliary soldiers filled specialist roles. Archers and slingers added much-needed ranged firepower to the legionnaires' javelins. Units of dedicated medical personnel were used to retrieve and tend to the Roman wounded. While legionnaires were capable of building a fort or mounting a siege when needed, many of the army's engineers were also auxiliaries, including those who tended the catapults and other siege engines.