Which doctrine was established as part of the United States' response during the Cold War?

The Truman Doctrine was a significant aspect of the United States' response during the Cold War, articulated in 1947. It emphasized the U.S. commitment to containing the spread of communism by providing political, military, and economic assistance to countries resisting Soviet influence. Initially, this doctrine aimed to support Greece and Turkey in their struggles against potential communist takeovers, thereby establishing a broader policy of containment that shaped American foreign policy for decades.

In contrast, the Marshall Plan, while also a post-World War II initiative, primarily focused on economic recovery in Europe and does not directly address the ideological battle against communism. The Roosevelt Corollary, formulated earlier in the early 20th century, dealt with U.S. intervention in Latin American countries and was not a response to the Cold War. Lastly, the Monroe Doctrine set a precedent for U.S. intervention in the Americas but predates the Cold War by over a century and was aimed at preventing European colonialism in the Western Hemisphere rather than directly addressing Cold War tensions. Thus, the Truman Doctrine is distinctly the cornerstone of U.S. strategy for countering the Soviet threat during the Cold War era.