Which doctrine asserted U.S. opposition to European colonialism in the Americas?

The Monroe Doctrine is the correct answer because it was a key policy articulated by President James Monroe in 1823, which established a clear message that the Western Hemisphere was no longer open to European colonization. This doctrine asserted that any attempt by European nations to interfere in the affairs of countries in the Americas would be viewed as an act of aggression toward the United States. The Monroe Doctrine was significant as it aimed to protect the newly independent nations of Latin America from European domination, demonstrating a shift in U.S. foreign policy toward greater intervention in Western Hemisphere affairs.

The other options represent distinct policies with different focuses. The Roosevelt Corollary, for instance, was an extension of the Monroe Doctrine that justified American intervention in Latin America under the guise of stabilizing the region and ensuring that European nations did not exploit local situations. The Truman Doctrine, established in the context of post-World War II geopolitics, was primarily concerned with containing communism rather than addressing colonialism. The Adams Doctrine, related to John Quincy Adams, generally refers to earlier policies that didn’t specifically encapsulate the strong anti-colonial stance established by the Monroe Doctrine. Thus, the Monroe Doctrine stands out as the defining statement against European colonialism in the Americas.