Civilian control

Article II Section 2 of the Constitution designates the President as "Commander in Chief" of the Army, Navy and state militias.^[2] The President exercises this supreme command authority through the civilian Secretary of Defense, who by federal law is the head of the department, has authority direction, and control over the Department of Defense, and is the principal assistant to the President in all matters relating to the Department of Defense.^[3] The Secretary's principal deputy is the equally civilian Deputy Secretary of Defense who is delegated full powers to act for the Secretary of Defense. The Office of the Secretary of Defense (OSD) is the Secretary and Deputy Secretary's civilian staff, which includes several Under Secretaries and Assistant Secretaries of Defense with functional oversight responsibilities. The Secretaries of the Military Departments (i.e. Secretary of the Army, Secretary of the Navy, and Secretary of the Air Force) are subordinate to the Secretary of Defense. They have the authority under Title 10 of the United States Code to conduct all the affairs of their respective departments (Department of the Army, Department of the Navy, and Department of the Air Force) within which the military services are organized.^[4] Although subordinate to the Secretary of Defense, they are appointed by, and serve at the pleasure of, the President of the United States. As such, they cannot be removed from office by the Secretary of Defense, but can only be removed unilaterally by the President, or through impeachment by the Congress.

Historically, there have been challenges to civilian control. Most notably, during the Korean War, General Douglas MacArthur ignored civilian instructions regarding advancing troops toward the Yalu River, which triggered an introduction of massive forces from China. Also, on April 5, 1950, Representative Joseph William Martin, Jr., the Minority Leader of the United States House of Representatives, released copies of a letter from MacArthur critical of President Harry S. Truman's limited-war strategy to the press and read it aloud on the floor of the house.^[5] President Truman relieved MacArthur of command, and MacArthur then explored political options against Truman.^[6] The Revolt of the Admirals is another example in the same era of a challenge to civilian control.

DoD policies and directives protect the policy of civilian control by establishing strict limitations on military members' political activities. For example, DoD Directive 1344.10 prohibits active-duty members of the military from running for office or making political appearances in uniform.^[7] However, enforcing this strict separation between the military and politics has been problematic. For example, over the years, many elected officials, including members of Congress, continued serving in the reserves while holding elected office. As another example, at a September 14, 2007, rally for Republican Presidential candidate John McCain in New Hampshire, seven on-duty uniformed Army personnel addressed the gathering.^[8] As another example, although DOD Directive 1344.10 prohibits political appearances by active-duty military members in uniform, Virginia Governor Bob McDonnell invited a uniformed Army Staff Sergeant to stand behind him during his televised Republican response to the 2010 State of the Union Address.^[9][10]

Defense Agencies

Defense Agencies are established as DoD Components by law, the President, or the Secretary of Defense to provide for the performance, on a DoD-wide basis, of a supply or service activity that is common to more than one Military Department when it is determined to be more effective, economical, or efficient to do so, pursuant to sections 101, 191(a), and 192 of Title 10 of the United States Code or when a responsibility or function is more appropriately assigned to a Defense Agency. Pursuant to section 191(b) Title 10, such organizations are designated as Defense Agencies. Each Defense Agency operates under the authority, direction, and control of the Secretary of Defense, through a Principal Staff Assistant in the Office of the Secretary of Defense.

On 1 March 2003 the Secretary of Defense lost Executive Agent responsibilities for the National Communications System, which was moved to the Department of Homeland Security. However the NCS still centralized its activities within the Department of Defense, since the human resources required by NCS (example: Military Departments) still reside within the Department of Defense, or for retention of practical maintenance. The NCS was finally disestablished in 2012.

Department of Defense Field Activities

Department of Defense Field Activities are established as DoD Components by law, the President, or the Secretary of Defense to provide for the performance, on a DoD-wide basis, of a supply or service activity that is common to more than one Military Department when it is determined to be more effective, economical, or efficient to do so, pursuant to sections 101, 191(a), and 192 of Title 10 of the United States Code. Pursuant to section 191(b) of Title 10, such organizations are designated as DoD Field Activities. Each DoD Field Activity operates under the authority, direction, and control of the Secretary of Defense, through a Principal Staff Assistant in the Office of the Secretary of Defense.

Military Departments