Which policy declared U.S. opposition to European interference in the Americas?

The Monroe Doctrine, articulated in 1823, established a significant principle in American foreign policy by declaring that any attempts by European nations to colonize or interfere in the affairs of countries within the Americas would be viewed as acts of aggression, necessitating U.S. intervention. This doctrine was rooted in the idea of protecting the newly independent nations of Latin America from European imperial ambitions, asserting a sphere of influence for the United States in the Western Hemisphere.

The policy not only aimed to safeguard these nations but also signaled a shift in international relations, establishing the Americas as a zone where European powers could no longer exert control or influence as they had in previous centuries. This was a foundational moment that underscored U.S. intentions and responsibilities toward its neighbors in the context of rising nationalism and imperialism of the time.

In contrast, the Roosevelt Corollary built upon the Monroe Doctrine later by justifying U.S. intervention in Latin America, particularly in matters of debts and economic instability. Manifest Destiny refers to the 19th-century belief that the U.S. was destined to expand across the North American continent, while the Open Door Policy was focused on promoting equal trading rights in China, not on European interference in the Americas. Thus, the Monroe Doctrine specifically