The word foederatus 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 billetted on local landowners, which came to be identical to being allowed to settle on Roman territory.
In 376 the Visigoths asked emperor Valens to allow them to settle on the southern bank of the Danube river, and were accepted into the empire as foederati. Two years later the Visigoths rose into 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 were 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 Germanic commander Odoacer deposed the last Roman emperor Romulus Augustulus.
The word federation is derived from the word foederati.