U.S. Department of Defense
The armed forces of the United States of America, along with the civil servants supporting them, form the Department of Defense (also called U.S. Department of Defense and abbreviated to DoD). It is led by a civilian Secretary of Defense appointed by the President and confirmed by the Senate.
Secretary of Defense
National Command Authority
The President and Secretary of Defense, or their successors, constitute the National Command Authority (NCA). Only the NCA can order the use of nuclear weapons. Military units in the field are almost always authorized, by classified rules of engagement, to use force in self-defense. There has been a conflict, since President Theodore Roosevelt sent out the "Great White Fleet", between Presidentialand Congressional authority to order troops to take offensive action. This conflict comes from the Constitutionally mandated authority of the Congress "to make war" and the President's authority as commander-in-chief. 
Office of the Secretary of Defense
A variety of offices, under officials of rank of Deputy, Assistant, or Deputy Assistant Secretary of Defense, report to the Secretary of Defense. The second highest official is the Deputy Secretary of Defense.
The uniformed, armed services of the United States are under two lines of authority, both descending from the NCA.
Preparation for operations
Traditional service branches:
- United States Army : Branch of the United States Armed Forces with the principal responsibility of conducting large-scale ground combat
- United States Navy : Branch of the U.S. armed forces] responsible for combat on, over, and under water.
- United States Air Force : Branch of the U.S. armed forces responsible for land-based aircraft, as well as land-based intercontinental ballistic missiles
- United States Marine Corps : Branch of the U.S. armed forces serving as elite fighters on land and aboard sea-going amphibious warfare ships.
prepare military units and personnel for operations. The United States Coast Guard, while formally under the Department of Homeland Security rather than the Department of Defense, may be brought under Navy authority. In this line of command, the services report to the professional head of their service:
- Army: Chief of Staff of the Army (CSA)
- Navy: Chief of Naval Operations (CNO)
- Air Force: Chief of Staff of the Air Force (CSAF)
- Marine Corps: Commandant of the Marine Corps(CMC), who, for administrative and budgetary matters, report to a service secretary, at a rank equivalent to an Assistant Secretary of Defense:
- Army: Secretary of the Army
- Navy and Marine Corps: Secretary of the Navy
- Air Force: Secretary of the Air Force
Deployment and operations
- United States Central Command
- United States European Command
- United States Joint Forces Command
- United States Northern Command
- United States Pacific Command
- United States Special Operations Command
- United States Strategic Command
- United States Southern Command
- United States Transportation Command
In 1789 the new U.S. government created a War Department and in 1798 a Navy department, each headed by a cabinet-level secretary. This arrangement reflected the military capabilities of the time (land and sea).
The National Security Act of 1947, created the "National Military Establishment", which joined together the War Department, Department of the Navy and the newly independent Air Force. Its name was changed in 1949 to Department of Defense. The Congressional Act created the Office of the Secretary of Defense, and established civilian positions as the heads of each branch of the military. Secretaries of the Army, Navy, and Air Force were established, each carrying the equivalent rank of Assistant Secretary of Defense. The United States Marine Corps remained a component reporting to the civilian Secretary of the Navy.
The 1947 act also created the National Security Council, Joint Chiefs of Staff, United States Air Force and Central Intelligence Agency. The Air Force celebrates its "birthday" each year based on the date the National Security Act was signed into law.
In February 2008 President George W. Bush sent Congress a DoD budget request is for $515.4 billion – a $35.9 billion increase over the 2008 level. The total is about 3.4% of U.S. gross domestic product. The total federal budget request for fiscal 2009 is $3.1 trillion. The budget funds the operations, training, recruiting and equipping of 2.2 million personnel in the Defense Department. Bush's budget allocates $140.7 billion for the Army, $149.2 billion for the Navy and Marine Corps and $143.8 billion for the Air Force. The House passed a revised budget by a voice vote in May, moving the bill to the Senate. In the House version appropriations would increase 3.25% to $602 billion, with $70 billion targeted for ongoing operations in Iraq and Afghanistan. Further funds for the war are tied up in supplemental spending bills. The DoD's Fiscal Year 2010 budget was roughly $700B, which consists of the baseline budget plus funds to fight the Afghanistan and Iraq wars. 
- Constitution of the United States of America Article I, Section 8
- Constitution, Article II, Section 2
- National Security Act of 1947.
- Jim Garamone, "Bush Delivers $515.4 Billion Defense Budget Request to Congress," American Forces Press Service Feb. 4, 2008
- Sandra Irwin, "Five Key Questions about the Defense Budget", National Defense Industrial Association August 2010