Foederatus, early in the history of the Roman Republic, identified one of the tribes bound by treaty (foedus), who were neither Roman colonies nor had they been granted Roman citizenship (civitas) but were expected to provide a contingent of fighting men when trouble arose. The Latini were considered blood allies, but the rest were federates or socii. The term is the root of the modern term federalism.
During the Roman republic, the friction between these treaty obligations without the corresponding benefits of Romanity led to the Social War between Romans, with a few close allies, and the disaffected Socii. A law of 90 BCE (Lex Julia) offered Roman citizenship to the federate states that accepted the terms. Not all cities (e.g. Heraclea and Naples) were prepared to be absorbed into the Roman res publica. Other foederati lay beyond Italy: Gades in Spain, and Massilia (Marseilles).
Later the term foederati was extended by the Roman practice of subsidizing entire barbarian tribes — which included the Attacotti, Franks, Vandals, Alans and, best known, the Visigoths — in exchange for providing soldiers to fight in the Roman armies. Alaric began his career leading a band of Gothic foederati.
The word federations came from the Latin word foedus, which indicated a solemn binding treaty of mutual assistance between Rome and another nation for perpetuity. At first, the Roman subsidy took the form of money or food, but as tax revenues dwindled in the fourth and fifth centuries, the foederati were billeted on local landowners, which came to be identical to being allowed to settle on Roman territory. Large local landowners living in distant border provinces (see "marches") on extensive, largely self-sufficient villas, found their loyalties to the central authority further compromised in such situations. Then, as loyalties began to fractionate and become more local, the Empire began to crumble into smaller and smaller territories.
The Franks became foederati in 358, when Julian the Apostate let them keep the areas in northern Gaul, which had been depopulated during the preceding century. Roman soldiers defended the Rhine and had major armies a 100 miles south and west of the Rhine. Frankish settlers were in the area north and east of the Romans and helped with the Roman defense. The breach of the Rhine borders in the winter of 406 and 407 made an end to the Roman presence at the Rhine. Both the Romans and the Franks were defeated by Vandals and Alans.
In 376 certain Goths asked Emperor Valens to allow them to settle on the southern bank of the Danube river, and were accepted into the empire as foederati. Later that year the Goths rose in rebellion and defeated the Romans in the Battle of Adrianople. The serious loss of military manpower forced the Roman Empire to rely more on foederati.
The loyalty of the tribes and their leaders was not reliable and in 395 the Visigoths, this time under the lead of Alaric, once again rose in rebellion. One of the most powerful Late Roman generals, a Vandal called Stilicho, was born of parents who were from the foederati.
By the fifth century, Roman military strength was almost completely based upon foederati units. In 451, Attila the Hun was defeated only with help of the foederati (who included the Visigoths and Alans). The foederati delivered the fatal blow to the dying Roman Empire in 476 when their commander Odoacer deposed the last Western Roman emperor Romulus Augustulus.
