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http://en.wikipedia.org/wiki/Monroe_Doctrine
The Monroe Doctrine is a policy of the United States introduced on December 2, 1823. It stated that further efforts by European nations to colonize land or interfere with states in North or South America would be viewed as acts of aggression, requiring U.S. intervention.[1] The Doctrine noted that the United States would neither interfere with existing European colonies nor meddle in the internal concerns of European countries. The Doctrine was issued at a time when nearly all Latin American colonies of Spain and Portugal had achieved independence from the Spanish Empire (except Cuba and Puerto Rico) and the Portuguese Empire. The United States, working in agreement with Britain, wanted to guarantee no European power would move in.[2]
President James Monroe first stated the doctrine during his seventh annual State of the Union Address to Congress. It became a defining moment in the foreign policy of the United States and one of its longest-standing tenets, and would be invoked by many U.S. statesmen and several U.S. presidents, including Theodore Roosevelt, John F. Kennedy, Lyndon B. Johnson, Ronald Reagan and others.
The intent and impact of the Monroe Doctrine persisted with only minor variations for almost two centuries. Its primary objective was to free the newly independent colonies of Latin America from European intervention and control that would make the New World a battleground for the Old. The doctrine put forward that the New World and the Old World were to remain distinctly separate spheres of influence, for they were composed of entirely separate and independent nations.