What doctrine warned European powers against colonization in the Americas?

The Monroe Doctrine is the historical policy that explicitly warned European powers against further colonization and intervention in the Americas. Enacted in 1823, this doctrine articulated that any attempts by European nations to colonize or interfere with states in North or South America would be viewed as acts of aggression, requiring U.S. intervention. It was a significant assertion of U.S. foreign policy aimed at preventing European powers from extending their influence in the Western Hemisphere and ensuring the region remained largely free from European colonialism.

This doctrine shaped U.S. foreign relations throughout the 19th century and beyond and established the foundation for American influence in the region. The other doctrines mentioned—Truman, Eisenhower, and Marshall—are related to different aspects of U.S. foreign policy, such as post-World War II containment strategies or economic aid, but they do not pertain to the issue of colonization in the Americas.