"The press hailed the National Security Act of 1947, public law 80-253, as a major accomplishment. Headlines called it a 'Unification Bill,' although it fell far short of merging the Armed Forces. In fact, it created an independent Air Force and preserved the autonomy of the Army, the Navy, and the Marine Corps. The new law did not even create the Department of Defense--only the awkwardly named National Military Establishment headed by a Secretary of Defense with just three special assistants. The secretary had only limited power to 'establish general policies and programs' and 'exercise general direction, authority, and control' over the service departments. In 1949, amendments to the law gave the position more power and created a regular Department of Defense. However, there have been few other significant changes in the 60 years since Truman signed the original bill. The story behind the act is a tale of bitter interservice rivalry, clever alliance building with Congress, clashing ambitions--and, yes, a desire to strengthen America's defenses so it could exert global leadership and counter the emerging Soviet threat."
Military Review: http://usacac.leavenworth.army.mil/CAC/milreview/
Military Review (May-June 2008), p.13-20