The Monroe Doctrine, articulated in 1823, was significant in U.S. history as it established the Western Hemisphere as a sphere of American influence, warning European nations against further colonization and intervention. The doctrine asserted that any attempt by European powers to control or interfere with states in the Americas would be viewed as acts of aggression, prompting U.S. intervention. This policy marked a turning point in U.S. foreign relations, reflecting the growing confidence of the young nation while also laying the groundwork for future American expansion and interventionism in Latin America.