How Anicent Rome's Citizenship Used to Work

Romans only recruited citizens of Rome. They did not recruit from conquered people.

They used mercenaries as auxiliaries, but they were always their own military units with their own commanders, who negotiated terms with Roman generals. Romans never made a non-Roman person a legionaries of a core Roman units.

Close to Roman collapse, they expanded citizenship to all the conquered people, but that did not save the country.

They were not generous about citizenship. Romans had clear distinction between citizens and aliens. Sure, empire was all around the Mediterranean, but most of its inhabitants were considered foreign aliens under Roman Law.

Only people born in City of Rome itself and to parents with Roman Citizen status would be citizens themselves.

Right and Privileges of citizens only applied within city limits as well. Outside the city it was a different legal jurisdiction.

Areas south of Rubicon in Italy were organized similar to Rome itself, but they had their own governments, citizenship and so on. They were like foreign countries, with whom Rome had diplomatic relationship and a military alliance. Unlike NATO, Rome was sole power responsible for defense of them all and the rest simply paid Romans for protection.

These independent Italian tribes arise first, then they established relationship with Rome.

In Greece there were city-states that used to be completely independent, but then ended up allying with fellow city-states for mutual defense until two military alliances emerged: Achaean League and Aetolian League.

Rome achieved something similar with other Italian city states, but there was no rival league like in Greece. They were all part of Roman League, but not citizens of Rome.

Areas north of Rubicon and outside Italy were outside of any legal jurisdiction. Roman military could do whatever they wanted there. At first, they invaded, pillaged and captured inhabitants to turn into slaves or gladiators. Later they established a military administration there. That run as tyrannical dictatorship with general running provinces as they saw fit and using military force to enforce their rule.

Senate could appoint and remove such governors. However, when their tried to terminate Caesar's tenure in Gallia, he staged a Coup and then Rome itself became a dictatorship. Then Senate assassinated him to restore rule of law.