答案:The Monroe Doctrine is a United States policy introduced on December 2, 1823, which stated that further efforts by European countries to colonize land or interfere with states in the Americas would be viewed by the United States as acts of aggression requiring US intervention The Monroe Doctrine asserted that the Western Hemisphere was not to be further colonized by European countries. The Doctrine was issued at the time when many Latin American countries were on the verge of becoming independent from Spain US 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.