Foederati (ˌfɛdəˈreɪtaɪ, singular: foederatus ˌfɛdəˈreɪtəs) were peoples and cities bound by a treaty, known as foedus, with Rome. During the Roman Republic, the term identified the socii, but during the Roman Empire, it was used to describe foreign states, client kingdoms or barbarian tribes to which the empire provided benefits in exchange for military assistance. The term was also used, especially under the empire, for groups of "barbarian" mercenaries of various sizes who were typically allowed to settle within the empire.
In the early Roman Republic, foederati were tribes that were bound by a treaty (foedus ˈfiːdəs) to come to the defence of Rome but were neither Roman colonies nor beneficiaries of Roman citizenship (civitas). Members of the Latini tribe were considered blood allies, but the rest were federates or socii. The friction between the 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 (Heraclea and Naples). Other foederati lay outside Roman Italy such as Gades (Cádiz) and Massilia (Marseilles).
The term foederati had its usage and meaning extended by the Romans' practice of subsidising entire barbarian tribes such as the Franks, Vandals, Alans, Huns and the Visigoths, the last being the best known, in exchange for providing warriors to fight in the Roman armies. Alaric I began his career leading a band of Gothic foederati.
At first, the Roman subsidy took the form of money or food, but as tax revenues dwindled in the 4th and the 5th centuries, the foederati were billeted on local landowners, which became identical to being allowed to settle on Roman territory. Large local landowners living in distant border provinces (see "marches") on extensive villas, which were largely self-sufficient, found their loyalties to the central authority, which were already conflicted by other developments, further compromised in such situations.