[nwrep]

which was typically made up of non-Roman auxiliaries under Roman command.

This practice of hired mercenaries from other lands enlisted to fight in their army, pioneered by the Romans, was also deployed by the Ottoman Turks in their ranks of the Janissaries from Eastern Europe.

[SunkenCiv]

The advantage of Rome’s use of auxiliaries came from the fact that they were unwelcome among the local populations, so they wouldn’t have an option other than to follow Roman leadership. It’s an old idea, long predating Rome. The Assyrians used it, for example.