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 and asserting its influence in the Americas. It laid the groundwork for future American actions in the region and was invoked in various contexts, including during the Spanish-American War and the Cold War.