This position was created in 1947, when the Navy, Army, and newly created Air Force were merged into the new National Military Establishment. In the same massive reorganization, the Secretary of the Navy was changed to a non-Cabinet position placed under the Secretary of Defense and the Secretary of War was replaced by the Secretary of the Army, also a non-Cabinet position under the Secretary of Defense. In 1949, the National Military Establishment was renamed the Department of Defense, which remains the current name of the department.
A Deputy Assistant Secretary of Defense specialises in a particular set of defense issues; e.g., the Deputy Assistant Secretary of Defense (Civilian Personnel Policy) -- DASD(CPP), the Deputy Assistant Secretary of Defense (C3I Acquisition) DASD(C3IA), and so on.