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A second important development in the history of the continent in the 19th and especially in the 20th century was the participation of the North American nations in the movement manifest throughout the Western Hemisphere for economic cooperation, for the attainment of peace and mutual understanding, and for solidarity against potential aggressors. In this movement the United States played a leading part, starting in 1823 with the Monroe Doctrine—the proclamation of President James Monroe that the United States would not permit European control of territories in the Americas beyond that existing at the time. The only serious intracontinental conflict was the so-called Mexican War (1846-1848) between the United States and Mexico. During the 20th century a tendency toward mutual friendship developed among the nations of the Western Hemisphere, given form in 1910 with the establishment of the Pan-American Union. Almost all the nations of the Western Hemisphere either declared war on or broke diplomatic relations with the Central Powers in World War I (1914-1918) and with the Axis Powers in World War II (1939-1945).
One of the most important demonstrations of hemispheric solidarity was the Inter-American Defense Conference of 1947, which promulgated the Inter-American Treaty of Reciprocal Assistance—the Rio Treaty—which was signed by the United States, Mexico, and 17 Central and South American nations. The treaty provides for settlement of disagreements between nations of the Western Hemisphere, as well as for joint defense against aggression on the region extending from the Bering Sea to the South Pole. In 1948 the Organization of American States (OAS) was formed to implement the Rio Treaty and to serve as a collective security system.
Hemispheric cooperation was temporarily furthered by the Alliance for Progress, which was established in 1961. The alliance, which was accepted by the United States and 19 Latin American nations at Punta del Este, Uruguay, consisted of a ten-year development plan to raise the economic and social levels of the area and to strengthen its democratic institutions. After the original ten-year period, however, the alliance showed mixed results, and it gradually ceased to function.
The existence after 1959 of a Communist government in Cuba tended to complicate hemispheric activities. In 1962, at Punta del Este, the OAS voted to exclude Cuba from participation in the Inter-American system because of that nation's alignment with the countries of the Communist bloc.
The relations between the United States and Canada have been particularly friendly and cooperative since the War of 1812. No military installations aimed at defense against each other have existed since that time on the entire border between the two nations. The United States and Canada collaborated closely in the fight against the Axis Powers during World War II. In the postwar period, usually referred to as the era of the Cold War, the Canadian and American governments initiated plans for joint defense against possible aggression from the Soviet Union across the Arctic regions.
Mexico's serious internal strife from 1910 to 1920 and its nationalization of U.S. oil companies in 1938 plagued relations between the two nations during the first half of the 20th century. More recently, however, their relationship has been more friendly, as evidenced by the signing of the North American Free Trade Agreement (NAFTA), which encouraged trade among the United States, Mexico, and Canada.