treaty or compact contracted by ancient Rome with one or more allied states (foederati). The treaty contained various conditions establishing permanent friendly relations between the contracting parties. A foedus aequum was a bilateral agreement recognizing both parties as equals obliged to assist each other in defensive wars or when otherwise called upon, in perpetuity. A foedus iniquum defined Rome as superior, the second party being bound to assist Rome in offensive wars, thus limiting the ally's sovereignty. Foedera were negotiated by Roman commanders and confirmed by vote of the centuriate assembly (comitia centuriata). The compacts were inscribed on bronze tablets and either displayed in public or kept in temples or other public buildings.

The earliest known foedus is the Foedus Cassianum signed by the consul Spurius Cassius Vecellinus in 493 BC, which established a common army of defense between the Romans and the collective Latin states. The terms of the treaty are preserved in the work of the Greek historian Dionysius of Halicarnassus (Roman Antiquities), and the treaty was kept on public display, according to Cicero (Speech in Defense of Balbus). As Rome gained dominance over the Italian peninsula, the Latin, Etruscan, and Italian polities lost their status as equals, eventually becoming provinces of Rome. Similarly, earlier Roman treaties with states outside Italy were usually equal, but the growth of Roman power in the Mediterranean eventually led to the subordination of the foederati. In time they lost both their local autonomy and their freedom from payment of tribute; this was especially true in the western provinces of the empire (after 27 BC). Certain peoples and rulers, however, termed socii et amici populi Romani (“allies and friends of the Roman people”) were given intermediate status between complete autonomy and organization as a Roman province. They usually paid tribute to Rome, while Rome was spared the burden of direct rule—which meant not having to underwrite the cost of defensive forces that would be a drain on the Roman treasury. Such status, however, was a privilege that could be revoked at any time.