Rome recruited foreign auxiliaries to balance out the legion system's few weaknesses as well as to bolster their numbers. Rome used a lot of foreign cavalry as well as foreign missile units, like slingers, to support its heavy infantry. Rome often deployed foreigners to parts of the Empire far from their homelands to prevent them from deserting or revolting, and there are records of Germans being deployed as far south as Egypt. At times, when the praetorian guard and even the normal legions were considered untrustworthy Emperors and generals would employ foreign body guards on the theory that they were unfamiliar with political intrigue and their safety and payment was only ensured by their commander.
As time went on and fewer and fewer Romans joined the military Rome had to rely increasingly on foreign military forces who were more independent and powerful then they had been in the past. This eventually came to haunt them as foreigners with Roman military experience were alienated by periodic bouts of racism and ineptitude from the Emperor and often took up arms against Rome and used their knowledge and experience against them.
As time went on and fewer and fewer Romans joined the military Rome had to rely increasingly on foreign military forces who were more independent and powerful then they had been in the past. This eventually came to haunt them as foreigners with Roman military experience were alienated by periodic bouts of racism and ineptitude from the Emperor and often took up arms against Rome and used their knowledge and experience against them.