The Inter-American Treaty of Reciprocal Assistance ("TIAR" or "Rio Treaty") grew out of the perceived need to provide a collective security mechanism for the hemisphere during the run-up to the Second World War. Although the groundwork for the treaty was laid during the war, the treaty was not adopted until the Third Meeting of Consultation of the Ministers of Foreign Affairs held in Rio de Janeiro, Brazil in 1947. The treaty entered into force in 1948. Since then, it has served as one of the pillars of the collective hemispheric defense architecture.

The Rio Treaty is a "special treaty" within the meaning of Article 29 of the OAS Charter. Among other things, it defines the measures and procedures governing a collective response by the other states party when a state party suffers an armed attack or an aggression that is not an armed attack. The Treaty has been invoked on 19 occasions since its inception, the most recent being 1982.

The Rio Treaty provides for a collective security mechanism. Under its terms, an armed attack on one member is to be considered an attack on all. The treaty also provides for measures to address aggressions that are not armed attacks, as well as for extracontinental or intracontinental conflicts, or "any other fact or situation that might endanger the peace of America."

Not all OAS member states are parties to the Treaty. The United States is a party to the Rio Treaty, along with Argentina, The Bahamas, Bolivia, Brazil, Chile, Colombia, Costa Rica, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Trinidad and Tobago, Uruguay, and Venezuela. Cuba is also a party although its present government has been suspended from OAS participation since 1962.