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Early in the history of the Roman Republic, a foederatus identified one of the tribes bound by treaty (foedus //), 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 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).[clarification needed]
Later, the sense of the term foederati and its usage and meaning was extended by the Roman practice of subsidizing entire barbarian tribes — which included the Franks, Vandals, Alans and, best known, the Visigoths — in exchange for providing warriors to fight in the Roman armies. Alaric 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 5th 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, already conflicted by other developments, further compromised in such situations. Then, as these loyalties wavered and became more local, the Empire began to devolve into smaller territories and closer personal fealties.
The Franks became foederati in 358 AD, 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 100 miles (160 km) south and west of the Rhine. Frankish settlers were established in the areas north and east of the Romans and helped with the Roman defense by providing intelligence and a buffer state in place. The breach of the Rhine borders in the frozen winter of 406 and 407 made an end to the Roman presence at the Rhine when both the Romans and the allied Franks were overrun by a tribal migration en masse of Vandals and Alans.
In 376 AD, some of the 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. These same Goths then rose in rebellion and defeated the Romans in the Battle of Adrianople in 378 AD. The critical ensuing loss of military manpower forced the Western Roman Empire to rely much more on foederati levies thereafter.
The loyalty of the tribes and their chieftains was never 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 both from the ranks of the foederati.
By the 5th century, lacking the wealth needed to pay and train a professional army, Western Roman military strength was almost entirely reliant upon foederati units. In 451 AD, Attila the Hun was defeated only with help of the foederati (who included the Visigoths and Alans), and the foederati would deliver the fatal blow to the dying nominal Western Roman Empire in 476 AD, when their commander Odoacer deposed the usurper Western Emperor, Romulus Augustulus, and sent the imperial insignia back to Constantinople with the Senate's request that the 81 year old West/East sub-division of the empire be abolished.
- George Long, "Foederati civitates" (English). An essay by a 19th-century Roman law scholar.
- Harry Thurston Peck, Harpers Dictionary of Classical Antiquities, 1898: Foederati