U.S. foreign-policy statement first enunciated by Pres.
James K. Polk to discourage Spain and Britain from establishing footholds in Oregon, California, or on Mexico's Yucatán Peninsula. In 1865 the U.S. massed troops on the Rio Grande to back up demands that France withdraw from Mexico. In 1904 Pres. Theodore Roosevelt added the Roosevelt Corollary, stating that in the event of flagrant wrongdoing by a Latin America n state, the U.S. had the right to intervene in its internal affairs. As the U.S. became a world power, the Monroe Doctrine came to define the Western Hemisphere as a U.S. sphere of influence . See also {{link=Good Neighbour Policy">Good Neighbor Policy .