Foederati

Foederatus (/ˌfɛdəˈreɪtəs/ in English; pl. foederati /ˌfɛdəˈreɪtaɪ/) was any one of several outlying nations to which ancient Rome provided benefits in exchange for military assistance. The term was also used, especially under the Roman Empire for groups of "barbarian" mercenaries of various sizes, who were typically allowed to settle within the Roman Empire.

Early in the history of the Roman Republic, a foederatus identified one of the tribes bound by treaty (foedus /ˈfiːdəs/), who were neither Roman colonies nor beneficiaries of Roman citizenship (civitas) but were expected to provide a contingent of fighting men when trouble arose, thus were allies. The Latini tribe were considered blood allies, but the rest were federates or socii. The friction between these treaty obligations without the corresponding benefits of Romanity led to the Social War between the Romans, with a few close allies, and the disaffected Socii. A law of 90 BC (Lex Julia) offered Roman citizenship to the federate states that accepted the terms. Not all cities were prepared to be absorbed into the Roman res publica (e.g. Heraclea and Naples). Other foederati lay beyond Italy: Gades in Spain, and Massilia (Marseilles).

...
Wikipedia