Monroe Doctrine, the United States policy that opposes foreign intervention in the affairs of the Americas. Its basic principles were established by President James Monroe in a message to the U.S. Congress on December 2, 1823. The doctrine was broadened over the next 100 years to permit United States intervention in the affairs of Central and South American nations for protective purposes. This policy of unilateral intervention by the United States was repudiated in the 1930's. In 1938 the principle of united resistance to foreign encroachment was adopted by all the republics of the Pan American Union. The United States, however, continued to dominate the affairs of the Western Hemisphere.