United States Department of Defense
The United States Department of Defense, also referred to as the Department of War, is an executive department of the U.S. federal government charged with coordinating and supervising the U.S. Armed Forces—the Army, Navy, Marines, Air Force, Space Force, and, for some purposes, the Coast Guard—and related functions and agencies. Headquartered at the Pentagon in Arlington County, Virginia, just outside Washington, D.C., the stated mission of the Department of Defense is "to provide the military forces needed to deter war and ensure our nation's security".
The Department of Defense is headed by the secretary of defense, a cabinet-level head who reports directly to the president of the United States. The president is commander-in-chief of the U.S. Armed Forces. Beneath the Department of Defense are three subordinate military departments: the Department of the Army, the Department of the Navy, and the Department of the Air Force. In addition, four national intelligence services are subordinate to the Department of Defense: the Defense Intelligence Agency, National Security Agency, National Geospatial-Intelligence Agency, and National Reconnaissance Office.
Other Department of Defense agencies include the Defense Advanced Research Projects Agency, Defense Logistics Agency, Missile Defense Agency, Defense Health Agency, Defense Threat Reduction Agency, Defense Counterintelligence and Security Agency, Space Development Agency and Pentagon Force Protection Agency, all of which are subordinate to the secretary of defense. Additionally, the Defense Contract Management Agency is responsible for administering contracts for the Department of Defense. Military operations are managed by eleven regional or functional unified combatant commands. The Department of Defense also operates several joint services schools, including the Eisenhower School and the National War College.
As of November 2022, the department has over 1.4 million active-duty uniformed personnel in the six armed services. It also supervises over 778,000 National Guard and reservist personnel, and over 747,000 civilians, bringing the total to over 2.91 million employees.
Name
By the National Security Act of 1947, the Department of the Navy and the Department of War had a new single Secretary imposed over the top of their two previously independent Cabinet secretaries. The War Department also changed its name to the Department of the Army and split off the Department of the Air Force. The new Cabinet-level department was initially designated the National Military Establishment. On August 10, 1949, the National Security Act of 1947 was amended; the amendment renamed the NME the Department of Defense. The renaming is alleged to be due to the NME's pronunciation as "enemy".On September 5, 2025, President Donald Trump signed an executive order authorizing "Department of War" and "secretary of war" as secondary titles to the main titles of "Department of Defense" and "secretary of defense." The terms must be accommodated by federal agencies and are permitted in executive branch communications, ceremonial settings, and non-statutory documents. However, only an act of Congress can legally and formally change the department's name and secretary's title, so "Department of Defense" and "secretary of defense" remain legally official. Trump described his rebranding as an effort to project a stronger and more bellicose name and said the "defense" names were "woke".
History
Faced with rising tensions between the Thirteen Colonies and the British government, one of the first actions taken by the First Continental Congress in September 1774 was to recommend that the colonies begin defensive military preparations. In mid-June 1775, after the outbreak of the Revolutionary War, the Second Continental Congress, recognizing the necessity of having a national army that could move about and fight beyond the boundaries of any particular colony, organized the Continental Army on 14 June 1775. Later that year, Congress would charter the Continental Navy on 13 October, and the Continental Marines on 10 November.War Department and Navy Department
Upon the seating of the 1st U.S. Congress on 4 March 1789, legislation to create a military defense force stagnated as they focused on other concerns relevant to setting up the new government. President George Washington went to Congress to remind them of their duty to establish a military twice during this time. Finally, on the last day of the session, 29 September 1789, Congress created the War Department. The War Department handled naval affairs until Congress created the Navy Department in 1798. The secretaries of each department reported directly to the president as cabinet-level advisors until 1949, when all military departments became subordinate to the Secretary of Defense.National Military Establishment
After the end of World War II, President Harry Truman proposed the creation of a unified department of national defense. In a special message to the Congress on 19 December 1945, the president cited wasteful military spending and interdepartmental conflicts. Deliberations in Congress went on for months focusing heavily on the role of the military in society and the threat of granting too much military power to the executive. On 26 July 1947, Truman signed the National Security Act of 1947, which established the National Military Establishment and created the National Security Council, National Security Resources Board, United States Air Force, and the Joint Chiefs of Staff. The NME was placed under the control of the new post of secretary of defense.The National Military Establishment formally began operations on 18 September, the day after the Senate confirmed James V. Forrestal as the first secretary of defense. The National Military Establishment was renamed the "Department of Defense" on 10 August 1949, and absorbed the three cabinet-level military departments, in an amendment to the original 1947 law.
Under the Department of Defense Reorganization Act of 1958, channels of authority within the department were streamlined while still maintaining the ordinary jurisdiction of the Military Departments to organize, train, and equip their associated forces. The Act clarified the overall decision-making authority of the secretary of defense concerning these subordinate military departments. It more clearly defined the operational chain of command over U.S. military forces as running from the president to the secretary of defense, the service chief of the unified combatant commanders, and then to the unified combatant commanders. Also provided in this legislation was a centralized research authority, the Advanced Research Projects Agency, eventually known as DARPA. The act was written and promoted by the Eisenhower administration and was signed into law on 6 August 1958.
Organizational structure
The secretary of defense, appointed by the president with the advice and consent of the Senate, is by federal law the head of the Department of Defense, "the principal assistant to the President in all matters relating to Department of Defense", and has "authority, direction, and control over the Department of Defense". Because the Constitution vests all military authority in Congress and the president, the statutory authority of the secretary of defense is derived from their constitutional authority. Since it is impractical for either Congress or the president to participate in every piece of Department of Defense affairs, the secretary of defense and the secretary's subordinate officials generally exercise military authority.The Department of Defense is composed of the Office of the Secretary of Defense, Joint Chiefs of Staff and Joint Staff, Office of the Inspector General, Combatant Commands, Military Departments, Defense Agencies and Department of Defense Field Activities, National Guard Bureau, and such other offices, agencies, activities, organizations, and commands established or designated by law, or by the president or by the secretary of defense. Department of Defense Directive 5100.01 describes the organizational relationships within the department and is the foundational issuance for delineating the major functions of the department. The latest version, signed by former secretary of defense Robert Gates in December 2010, is the first major re-write since 1987.