On this day, 78 years ago, September 2, 1947 (July 18, 1947 in the lunar calendar), the Treaty of Rio de Janeiro was signed. On September 2, 1947, the United States and 18 Latin American countries jointly signed the "Treaty of Mutual Assistance between American States", known as the "Treaty of Rio de Janeiro." This treaty was promoted by the United States since early 1947 and was promoted by further intensification of planning after the announcement of the "Truman Doctrine". It is further developed on the basis of the Chaptepek Protocol and has a background of the "Cold War". The most important clause of this treaty stipulates that "an attack on any American country will be regarded as an attack on all American countries," and therefore the parties are obliged to take assistance measures to the directly affected countries and to adopt collective measures through appropriate institutions." The possible actions listed in the treaty include the use of force. It is worth mentioning that the treaty invokes Article 51 of the United Nations Charter on regional defense rights, which happened to be initiated by the United States to legalize the Pan-American system. It turned out that the Dumbarton Oaks bill did not include this clause and stipulated that "no enforcement action shall be taken in accordance with regional arrangements or by regional agencies without the authorization of the Security Council." After the adoption of the Chaptepik Protocol, it obviously conflicts with this clause. Therefore, at the San Francisco meeting, the United States supported Latin American countries in opposing this rule. In the end, Vandenberg proposed a compromise plan that recognized the supreme authority of the Security Council and allowed the Pan-American system to play its role. It was finally adopted in the form of Article 51 of the United Nations Charter: When any Member State of the United Nations is attacked by force, this Charter shall not be deemed to prohibit the exercise of the natural right of individual or collective self-defense until the Security Council takes the necessary measures to maintain international peace and security. Measures taken by a Member State in the exercise of this right of self-defense shall be reported immediately to the Security Council, and such measures shall not in any way affect the right and responsibility of that Member State to take such action as it deems necessary at any time in accordance with this Charter to maintain or restore international peace and security. Now "Rio" is quoting this clause in turn, which is actually manipulated by the United States. The importance of "Rio" is that it is the first collective defense treaty led by the United States, setting a precedent for a series of defense groups such as "NATO" in the future. Its scope of application is not limited to Latin American countries. The draft proposed by the United States was originally vague. Because most Latin American countries were worried about assuming obligations for the United States 'conflicts in various regions of the world, they insisted on delineating the scope, that is, including Greenland, Aleutian Island, Bermuda and other areas. In addition, after the "Truman Doctrine", revolutions led by the Communist Party in American countries were also included in foreign aggression. This was also one of the purposes of the United States actively promoting the treaty.