The Monroe Doctrine was a U.S. policy established in 1823 by President James Monroe. It stated that any European intervention in the Americas would be viewed as a threat to U.S. security. The doctrine aimed to prevent further colonization and interference by European powers in the Western Hemisphere. The Monroe Doctrine was significant in shaping U.S. foreign policy. It asserted that the Americas were no longer open to European colonization and emphasized the importance of independence for Latin American countries. Over time, it became a cornerstone of American diplomacy and influenced relations with both Europe and neighboring nations.