The defense structure of the United States began as a simple concept: in 1789 Congress established the War Department to conduct all military operations by the new nation. However, nine years later (in 1798) Congress separated the naval from the land forces by establishing a separate Department of the Navy. The Secretaries of War and the Navy were both Cabinet members who reported directly to the President, who personally coordinated issues between them. The Navy's mission was to fight on the seas, the Army's to fight on land, and the Marines were an amphibious force that served with the Navy. This arrangement remained largely in effect until 1903, when the Joint Board of the Army and Navy was established to coordinate the actions of the Army and the Navy. During the First World War the need to orchestrate large scale operations involving coordination between the services became imperative.

After the First World War Congress considered merging the separate Services into a single "Department of National Defense," primarily for fiscal reasons. In spite of periodic Congressional interest in consolidating the services, no action was taken. However, during the Second World War it became necessary to rethink the whole issue of command and control of the Armed Forces. President Franklin D. Roosevelt (in concert with England's Prime Minister Winston Churchill), acting as Commander in Chief of the Armed Forces, established the Combined Chiefs of Staff as the supreme military body for strategic direction of the Anglo-American war effort. In 1942 a "unified high command" known as the U.S. Chiefs of Staff was established but functioned throughout the war without a formal charter. After the war the need for the same or a similar structure was evident, and this is why the Joint Chiefs of Staff was created under the National Security Act of 1947.

The National Security Act of 1947 created a single Secretary of Defense as head of the "National Military Establishment" and gave him Cabinet rank. The law provided that the Secretary of Defense would report directly to the President but left the separate Services intact. Congress also formally chartered the Joint Chiefs of Staff and authorized a Joint Staff to enable the Joint Chiefs to carry out their duties. In addition, a separate Air Force was created out of the Army Air Forces; the National Security Council was established to advise the President on all aspects of foreign and military policy; and, the Central Intelligence Agency (under the National Security Council) was created to coordinate all intelligence functions. 

President Harry S. Truman signed the National Security Act on July 26, 1947, establishing a Secretary of Defense whose primary task was to coordinate defense matters among the separate Services and to develop general policies for the Army, Navy, and Air Force (each of which was to remain an Executive department). On September 17, 1947, James Forrestal took the oath of office as the first Secretary of Defense, and on the following day the National Military Establishment came into being. In 1949 the National Security Act was amended to change the name of the National Military Establishment to the Department of Defense (this legislation also withdrew the Executive status from the military departments, which were thereafter to be administered under the authority and direction of the Secretary of Defense).

The Defense Department is grouped into Six broad entities:

• Office of the Secretary of Defense