United States Department of Defense

The United States Department of Defense (DoD), also referred to as the Department of War (DoW), 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 Department of Defense's stated mission 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 (NSA), National Geospatial-Intelligence Agency, and National Reconnaissance Office.

Other Department of Defense agencies include the Defense Advanced Research Projects Agency (DARPA), 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.

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 (NME). In 1949, the NME was renamed 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".

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.

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.