The Department of Defense (DOD) was established after World War II by the 1947 National Security Act (P.L. 80-253). At the time, some policymakers, including former President Truman, took the view that the different components of the U.S. military had been insufficiently integrated to wage World War II effectively. Congress's stated intent was to, inter alia, "provide a Department of Defense ... under the direction, authority, and control of the Secretary of Defense; ... provide more effective, efficient, and economical administration; ... and provide for the unified strategic direction of the combatant forces." Since September 5, 2025, DOD has been "using a secondary Department of War designation," under Executive Order 14347.
"One of the lessons which have most clearly come from the costly and dangerous experience of this war is that there must be unified direction of land, sea and air forces at home as well as in other parts of the world where our Armed Forces are serving."
President Harry S. Truman, "Message to Congress," December 19, 1945.
Over time, DOD has grown into one of the largest bureaucracies in the world, currently comprising more than 2.8 million servicemembers and civilians stationed in over 4,800 sites across the United States and around the globe. Congress, in its constitutional role, legislates both defense authorization and appropriations bills, and conducts oversight of DOD, through budget and posture hearings. The Senate provides advice and consent through the confirmation process for multiple DOD civilian officials, including the Secretary and Deputy Secretary, and military officers, including the Chairman of the Joint Chiefs of Staff, and officer promotion selection lists.
DOD's mission is "to provide the military forces needed to deter war and ensure our nation's security." It does so through five primary sets of institutions, each representing thousands of people in dozens of specific offices:
Title 10, Section 113, of the U.S. Code specifies that the Secretary of Defense exercises "authority, direction and control" over DOD. OSD assists the Secretary in exercising such authority in a variety of areas, including policy development, planning, resource management, fiscal management, and program evaluation. OSD political appointees and civilian employees also help provide civilian oversight of the military services and combatant commands.
The Joint Chiefs of Staff is the preeminent military advisory body in the U.S. national military establishment. Its membership consists of the six military service chiefs (Army, Navy, Air Force, Marine Corps, National Guard Bureau, and Space Force), the Chairman of the Joint Chiefs of Staff (CJCS), and the Vice Chairman of the Joint Chiefs of Staff (VCJCS). The JCS regularly convenes to formulate and provide military advice to the President, the National Security Council, and the Secretary of Defense.
According to Title 10, Section 151, of the U.S. Code, the CJCS is the principal military advisor to the President; the CJCS does not exercise command authority over U.S. military forces. The CJCS therefore has statutory responsibility to present military advice—as well as any dissenting views from other members of the JCS—to senior leaders in the U.S. national security establishment. Per the Joint Staff website, the JCS have no executive authority to command combatant forces. The Goldwater-Nichols Department of Defense Reorganization Act of 1986 (P.L. 99-433, 10 U.S.C. §162(b)) states, "Unless otherwise directed by the President, the chain of command to a unified or specified combatant command runs from the President to the Secretary of Defense; and from the Secretary of Defense to the commander of the combatant command."
The Joint Staff supports the CJCS by developing the unified strategic direction of the combatant forces, their operations under unified command, and their integration into a team of land, naval, and air forces. The Joint Staff comprises officers and noncommissioned officers from the five military Services, the Reserve Component, and DOD civilians.
There are three military departments: the Army, Navy and Air Force. The Marine Corps is part of the Department of the Navy. The Space Force is part of the Department of the Air Force. The departments organize, train, and equip the military forces. According to Title 10, Sections 7013, 8013, and 9013, of the U.S. Code, a civilian Service Secretary leads each department; each Service Secretary "is responsible for, and has the authority necessary to conduct, all affairs of the Department." Each service has a service chief (a senior military officer in the grade of O-10, General or Admiral) who reports to and performs duties under the authority, direction, and control of the department's Secretary, and is a member of the Joint Chiefs of Staff.
The Departments and Services are to organize, train, and equip units for operations in each combat domain. The Army is to provide forces for the land domain. The Navy is to provide forces for operations at sea. The Marine Corps is to provide expeditionary, sea-based, integrated air-ground units on land and at sea. The Air Force is to provide forces for aerial operations. The Space Force is to provide forces and integrate space operations into combatant commands' joint operations.
The Unified Combatant Commands, or "COCOMs," are the principal mechanism through which DOD conducts its joint (i.e., multi-Service) global operations, by employing assigned Service units. It is primarily the COCOMs' operational capability needs that drive the development of military requirements across the department. There are seven regionally focused COCOMs.
There are also four "functional" COCOMs.
Title 10, Section 191, of the U.S. Code authorizes the Secretary of Defense to establish agencies that provide for the performance of a supply or service activity that is common to more than one military department when the Secretary deems it more effective, economical, or efficient than existing methods. DOD calls an organization established under this authority either a Defense Agency or a DOD Field Activity. According to DOD, there are 20 Defense Agencies and eight DOD Field Activities. Eight of the Defense Agencies are also designated Combat Support Agencies per Title 10, Section 193, of the U.S. Code. The designation requires that the CJCS report at least biannually to the Secretary of Defense and the congressional defense committees on each agency's ability to support operating forces in time of war or threat to national security.
DOD Strategy, Resources, and Organization. The 2026 National Defense Strategy prioritizes homeland defense and the Western Hemisphere, focuses on the Indo-Pacific, and highlights preparedness for war on multiple fronts. Congress may assess whether DOD's organizational structure and resourcing (trending toward $1.4 trillion annually over five years) is appropriate to execute the strategy across all combat domains. Some Members have proposed a separate "Cyber Service" under the Department of the Army. Members have also proposed creation of a 4-star officer-led "Robotic and Autonomous Systems Combatant Command," reportedly inspired by Ukraine's military. Congress may debate, defer, or direct implementation of all or some of the aforementioned changes to meet operational imperatives.
DOD Management and Staffing. Some have criticized growth of DOD bureaucracy, including headquarters staffing. Congress has directed reviews of DOD organization and placed statutory limits on General and Flag officer billets. In 2025, DOD announced an effort to "strategically restructure" the civilian workforce, particularly at the headquarters level. Congress may monitor DOD's implementation of these initiatives to gauge any effects on business operations efficiency and mission readiness.
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CRS Products CRS Report R44757, Defense Primer: A Guide for New Members, by Bryce H. P. Mendez and Daniel M. Gettinger |