The National Security Act of 1947 (Pub.L. 80-253, 61 Stat. 495, enacted July 26, 1947) was a law enacting major restructuring of the United States government's military and intelligence agencies following World War II. The majority of the provisions of the act took effect on September 18, 1947, the day after the Senate confirmed James Forrestal as the first secretary of defense.The act merged the Department of the Army (renamed from the Department of War), the Department of the Navy, and the newly established Department of the Air Force (DAF) into the National Military Establishment (NME). The act also created the position of the secretary of defense as the head of the NME It established the United States Air Force under the DAF, which worked to separate the Army Air Forces into its own service. It also protected the Marine Corps as an independent service under the Department of the Navy. Aside from the unification of the three military departments, the act established the National Security Council and the Central Intelligence Agency, the latter of which is headed by the Director of Central Intelligence.The legislation was a result of efforts by Harry S. Truman beginning in 1944. President Truman proposed the legislation to Congress on February 26, 1947. The bill was introduced in the U.S. House of Representatives on February 28, 1947, and in the Senate on March 3, 1947. Senator Chan Gurney was the bill's sponsor. Senator Gurney, as chairman of the Senate Committee on Armed Services, led committee hearings for the bill from mid-March to early May. The bill passed in the Senate on July 9, 1947, and in the House on July 19, 1947. The Senate agreed to a related House resolution (80 H.Con.Res. 70) on July 16, 1947. The bill received bipartisan support and was passed in both chambers by voice vote. The National Security Act of 1947 was signed into law by President Truman on July 26, 1947, while aboard his VC-54C presidential aircraft Sacred Cow.

Harry S. Truman (May 8, 1884 – December 26, 1972) was the 33rd president of the United States, serving from 1945 to 1953. A lifetime member of the Democratic Party, he previously served as a US Senator from the State of Missouri from 1935 to 1945. He was chosen as incumbent President Franklin D. Roosevelt's running mate for the 1944 presidential election. Truman was inaugurated as vice-president in 1945 and served for less than three months until President Roosevelt died. Now serving as president, Truman implemented the Marshall Plan to rebuild the economy of Western Europe and established both the Truman Doctrine and NATO to contain the expansion of communism. He proposed numerous liberal domestic reforms, but few were enacted by the Conservative Coalition that dominated the Congress.

Truman grew up in Independence, Missouri, and during World War I fought in France as a captain in the Field Artillery. Returning home, he opened a haberdashery in Kansas City, Missouri, and was elected as a judge of Jackson County in 1922. Truman was elected to the United States Senate from Missouri in 1934 and gained national prominence as chairman of the Truman Committee, which was aimed at reducing waste and inefficiency in wartime contracts. Upon assuming the presidency, he was informed about the Manhattan Project. Truman authorized the first and only use of nuclear weapons in war against the Empire of Japan. Truman's administration engaged in an internationalist foreign policy by working closely with British Prime Minister Clement Attlee. Truman staunchly denounced isolationism. He energized the New Deal coalition during the 1948 presidential election and won a surprise victory against Thomas E. Dewey that secured his own presidential term.

After the onset of the Cold War, Truman oversaw the Berlin Airlift and Marshall Plan in 1948. When North Korea invaded South Korea in 1950, he lobbied for intervention from the United Nations in the Korean War. The Truman administration deployed forces in Korean War without congressional authorization and as the war stalemated his popularity fell. Domestically, his administration successfully guided the U.S. economy through the postwar economic challenges; the expected postwar depression never happened. In 1948, he proposed the first comprehensive civil rights legislation since Reconstruction. Congress failed to pass these measures which prompted Truman to issue Executive Orders 9980 and 9981 which ordered the desegregation of the armed forces and agencies of the federal government.

Corruption in the Truman administration became a central campaign issue in the 1952 presidential election. He was eligible for reelection in 1952, but with weak polls he decided not to run. Republican Dwight D. Eisenhower attacked Truman's record and won easily. Truman went into a retirement marked by the founding of his presidential library and the publication of his memoirs. It was long thought that his retirement years were financially difficult for Truman, resulting in Congress voting a pension for former presidents, but ample evidence eventually emerged that he amassed considerable wealth, some of it while still president. When he left office, Truman's administration was heavily criticized, though critical reassessment of his presidency has improved his reputation among historians and the general population.