Rio Treaty (Inter-American Treaty of Reciprocal Assistance), signed Sept. 2, 1947, and originally ratified by all 21 American republics. Under the treaty, an armed attack or threat of aggression against a signatory nation, whether by a member nation or by some other power, will be considered an attack against all (see Pan-Americanism). The treaty provides that no member can use force without the unanimous consent of the other signatories, but that other measures against aggressors may be approved by a two-thirds majority. It differs from previous inter-American treaties in that it is a regional treaty within a larger international organization; it recognizes the higher authority of the Security Council of the United Nations.
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