“The legions of the Roman Empire upto the early 3rd century AD consisted of Latin citizens of the Roman Empire only. Barbarian and foreign troops in the Imperial service served as auxilia and comprised almost all the archers(sagittarii) and the cavalry(alae), though the cavalry also included the equites(Italian nobles). The Antonine Decree of 212 AD issued by Emperor Caracalla, granted citizenship to all inhabitants of the Empire, resulting in the removal of the distinction between the legions and the auxilia. However, the Roman Empire faced a severe military and economic crisis in the 3rd Century AD, including the loss of most of Gaul(to the Alamanni) and the East to the Persians, inlcuding a loss of prestige due to the capture of Emperor Valerian at the Battle of Edessa (259 AD) by the Sassanids.
All these factors led to a socio-economic crisis resulting in the devaluation of Roman currency with the minting of the antoninianus, which had only 5% silver compared to its predecessor. Thus 20 times more money could be distributed with the same amount of silver. This resulted in rampant inflation, leading to the army’s recruits leading a remarkably meagre lifestyle as their salaries were now largely worthless.
This led to a marked decline in the number of citizens volunteering for the army, forcing the government to rely on large scale recruitment of barbarians from the provinces due to the shortage of manpower from Italy as a result of the currency devaluation and the plague of the 3rd Century.
In the Later Roman Empire(4th and 5th century AD) the Empire was witness to a mass migration of barbarians into the Empire. These tribes were allowed to settle on Imperial lands in exchange for military service leading to the creation of the term foederati, which is the progenitor of the feudal system. These foederati were largely led by their own tribal chieftains, which would later lead to further instability in the Empire, ultmately leading to its downfall when the foederatus commander, Odoacer deposed the Western Emperor, Romulus Augustus, thereby ending the Western Roman Empire.”