President Harry S. Truman signs the National Security Bill, which establishes the Department of Defense. As the Cold War heated up, the Department of Defense became the cornerstone of America's military effort to contain the expansion of communism.
In 1947, the National Security Act established the Cabinet-level position of secretary of defense, which oversaw a rather unwieldy umbrella military-defense agency known as the National Military Establishment. The secretary of defense, however, was just one of a number of military-related cabinet positions, including the pre-existing secretaries for the Army, Navy, and Air Force. The growing complexity of the Cold War, a war in which the mishandled application of military force could lead to a world war of cataclysmic proportions, convinced U.S. officials that the 1947 act needed to be revised.
In 1949, the National Security Bill streamlined the defense agencies of the U.S. government. The 1949 bill replaced the National Military Establishment with the Department of Defense. The bill also removed the cabinet-level status of the secretaries of the Army, Navy, and Air Force, who would henceforth be subordinate to the Secretary of Defense. The first person to hold this position was Louis Johnson. Finally, the bill provided for the office of chairman of the Joint Chiefs of Staff in an effort to bring to end to the inter-service bickering that had characterized the Joint Chiefs in recent years. World War II hero General Omar Bradley was appointed the first Chairman of the Joint Chiefs of Staff.
The National Security Bill of 1949 was the result of the realization that more coordination and efficiency were needed for America's military-defense bureaucracy, which had experienced tremendous growth during and after World War II. The Cold War was a new and dangerous kind of war for America, and the 1949 reorganization was recognition of the need for a different approach to U.S. defense.