What did the Monroe Doctrine say about future European colonization in the Americas?

The Monroe Doctrine, articulated by U.S. President James Monroe in 1823, established that the Western Hemisphere was no longer open to future colonization by European powers.12

The doctrine stipulated that any attempt by a European nation to colonize territory in the Americas or to interfere with the governments of the Western Hemisphere would be viewed by the United States as an act of aggression and a manifestation of an unfriendly disposition. While the United States pledged not to interfere with existing European colonies or the internal affairs of European states, it asserted that the New World and the Old World were distinct spheres that must remain separate.132

At the time of its announcement, the United States lacked the military strength to enforce this policy unilaterally and relied on the tacit support of the British fleet to deter potential European encroachment. Over the following decades, the doctrine was reinterpreted and expanded, eventually serving as a foundation for U.S. influence in the region.23