The U.S. president when NATO was founded in 1949 was Harry S. Truman. He was the 33rd president, serving from 1945 to 1953, and he played a key role in establishing NATO as part of his administration's policy to contain Soviet expansion in Europe after World War II. Truman officially signed the North Atlantic Treaty on August 24, 1949, after the treaty was initially signed by Secretary of State Dean Acheson on April 4, 1949