The Monroe Doctrine established the principle of U.S. foreign policy that opposed European colonialism in the Americas. Declared in 1823 by President James Monroe, it asserted that any intervention by external powers in the politics of the Americas would be viewed as a hostile act against the United States. The doctrine aimed to prevent further European expansion in the Western Hemisphere, thereby asserting U.S. interests and influence, and laid the groundwork for future American interventions in Latin America and the Caribbean.