Explain and evaluate 3 major reasons why and

explain and evaluate 3 major reasons why and
how the United States became the leader of the free world during the Cold War.
Link to a source I need to use in my paper:
https://history.defense.gov/Multimedia/Biographies/Article-View/Article/571266/george-c-marshall/
Also I need to use textbook info in my essay:
Document 26-1: General Marshall, For the Common Defense, 1945
Document 26-2: George F. Kennan, The Long Telegram, February 22, 1946
Document 26-3: NSC-68: U.S. Objectives and Prograbcims for National Security, 1950