Updated November 30, 2022
Defense Primer: The Department of Defense
The Department of Defense (DOD) was established after
provide civilian oversight of the military services and
World War II through the 1947 National Security Act. At
combatant commands to ensure that the Secretary’s and the
the time, some, including President Truman, took the view
President’s defense objectives are met.
that the different components of the U.S. military had been
insufficiently integrated to wage World War II effectively.
The Joint Chiefs of Staff (JCS)
The intention of the 1947 Act was therefore to create, for
The Joint Chiefs of Staff is the preeminent military
the first time, an integrated institution that combined the
advisory body in U.S. national security establishment. Its
Departments of War and Navy, and to establish a policy
membership consists of the six military service chiefs
architecture for overseeing the newly reorganized military
(Army, Navy, Air Force, Marine Corps, National Guard
apparatus. Over time, DOD has grown into one of the
Bureau, and Space Force), the Chairman of the Joint Chiefs
largest bureaucracies in the world, currently comprising
of Staff (CJCS), and the Vice Chairman of the Joint Chiefs
over 2.9 million service members and civilians stationed in
of Staff (VCJCS). The JCS regularly convenes to formulate
4,800 sites across the United States and around the globe.
and provide its best military advice to the President, the
Congress, in its constitutional role, legislates both
National Security Council, and the Secretary of Defense.
authorization and appropriations bills, as well as conducting
According to Title 10, U.S.C., §151, the Chairman is the
oversight on all of DOD. The Senate confirms multiple
principal military advisor to the President. The CJCS
military officers and certain DOD civilian officials.
therefore has statutory responsibility to present his or her
counsel—as well as any dissenting views from other
members of the JCS—to senior leaders in the U.S. national
“One of the lessons which have most clearly come
security establishment. Of note, although the CJCS plans,
from the costly and dangerous experience of this war
coordinates, and oversees military operations involving
is that there must be unified direction of land, sea and
U.S. forces, neither the CJCS nor the JCS has a formal role
air forces at home as well as in other parts of the
in the execution of military operations—a role instead
world where our Armed Forces are serving. We did
assigned to the unified combatant commanders.
not have that kind of direction when we were
attacked four years ago—and we certainly paid a high
The CJCS is supported by the Joint Staff, which assists in
price for not having it.”
developing the unified strategic direction of the combatant
President Harry S. Truman, “Message to Congress,”
forces, their operation under unified command, and their
December 19, 1945.
integration into an efficient team of land, naval, and air
forces. The Joint Staff is composed of approximately equal
numbers of officers from the Army, Navy, Marine Corps,
DOD’s purpose today is to provide the President with the
Air Force, and Space Force, as well as Department of
military forces needed to deter war and to protect the
Defense civilians. In practice, the Marines make up about
security of the country. It does so through five primary sets
20% of the number allocated to the Navy.
of institutions, each representing thousands of people and
The Military Departments
often hundreds of specific offices:
There are three military departments: the Army, Navy and
The Office of the Secretary of Defense;
Air Force. The Marine Corps, mainly an amphibious force,
is part of the Department of the Navy. The Space Force is
The Joint Chiefs of Staff and Joint Staff;
part of the Department of the Air Force. These departments
The Military Departments;
are responsible for training and equipping the military
forces utilized by the combatant commands; departments
The Unified Combatant Commands; and
therefore produce DOD’s “supply” of military forces and
The Defense Agencies.
equipment. Each department is led by a civilian service
secretary and supported by a service chief, both confirmed
These institutions are described below.
by the Senate. A service chief is a senior military officer
designated as the principal military advisor to a
The Office of the Secretary of Defense (OSD)
departmental secretary for matters relating to a specific
Title 10 U.S. Code Section 113 specifies that the Secretary
armed service.
of Defense exercises “authority, direction and control” over
DOD. The Office of the Secretary of Defense (OSD) assists
In terms of overall roles and responsibilities, The Army
the Secretary of Defense in exercising such authority over
trains and equips forces to provide ready, prompt, and
DOD. They do in a variety of areas, including policy
sustained land dominance across the full spectrum of
development, planning, resource management, fiscal
conflict as part of the joint force. The Navy maintains,
management, and program evaluation. OSD also helps
trains, and equips combat-ready maritime forces capable of
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Defense Primer: The Department of Defense
winning wars, deterring aggression, and maintaining
of the Joint Force to support persistent, networked and
freedom of the seas. The U.S. Marine Corps maintains
distributed Combatant Command operations and
ready expeditionary forces, sea-based and integrated air-
campaigns against state and non-state actors.
ground units for contingency and combat operations, and
U.S. Transportation Command provides air, land, and
the means to stabilize or contain international disturbance.
sea transportation to different components of DOD; and
The Air Force provides a rapid, flexible, and, when
necessary, lethal air capability that can deliver forces
U.S. Cyber Command directs, synchronizes, and
anywhere in the world in less than 48 hours. The Space
coordinates cyberspace planning and operations to
Force organizes, trains, and equips its forces for global
defend and advance national interests in collaboration
space operations.
with domestic and international partners.
Unified Combatant Commands
Defense Agencies
The Unified Combatant Commands, or “COCOMs,” are the
Title 10, U.S.C., §191 grants the Secretary of Defense
principal mechanism through which DOD conducts its
authority to establish agencies that provide for the
global operations. The COCOMs represent the “demand”
performance of a supply or service activity that is common
side of DOD, as it is primarily the COCOMs’ operational
to more than one military department, in instances where
needs that drive the development of military requirements
doing so is deemed more effective, economical, or efficient
across the department.
than existing structures. DOD refers to organizations
established under this authority as either Defense Agencies
There are seven regionally focused COCOMS:
or a DOD Field Activity. According to DOD’s FY2022
Agency Financial Report, there are 19 Defense Agencies (7
U.S. Africa Command, responsible for sub-Saharan
of which are also designated Combat Support Agencies
Africa;
pursuant to Title 10, U.S.C., §193, meaning that they are
U.S. European Command, responsible for all of Europe,
jointly overseen by the Chairman of the Joint Chiefs of
large portions of Central Asia, parts of the Middle East,
Staff), and 8 DOD Field Activities. These include the
and the Arctic and Atlantic Oceans;
Defense Security and Cooperation Agency, the Defense
Logistics Agency, and the Defense Commissary Agency,
U.S. Central Command, responsible for most of the
among others.
Middle East, parts of Northern Africa and west Asia,
and part of the Indian Ocean;
DOD Reform
U.S. Northern Command, responsible for defense of the
Managing an organization as large and complex as DOD
continental United States and coordination of security
presents a unique challenge to its senior leaders. Concerns
and military relationships with Canada and Mexico;
about the department’s efficiency, effectiveness, or both
have driven reform initiatives since the establishment of
U.S. Southern Command, responsible for Central
DOD itself. In the early 1980s, congressional concerns that
America, South America, and the Caribbean; and
the services had undue—and unhelpful—influence in the
U.S. Indo-Pacific Command, responsible for the Pacific
conduct of military operations led to the 1986 Goldwater-
Ocean, Southwest Asia, Australia, South Asia, and parts
Nichols Defense Reform Act. Thirty years later, the
of the Indian Ocean. It shares responsibility for Alaska
FY2017 National Defense Authorization Act (P.L. 114-
with U.S. Northern Command.
328) also sought to reform the way DOD is organized, with
the overall aim of improving the department’s agility and
U.S. Space Command provides space combat power for
strategic integration of its global assets.
the Joint/Combined force and develops joint warfighters
in, from, and through the space domain.
CRS Products
There are also four “functional” COCOMs:
CRS Report R44474, Goldwater-Nichols at 30: Defense Reform
and Issues for Congress, by Kathleen J. McInnis
U.S. Strategic Command, responsible for deterring
attacks on the United States and its allies and directing
the use of U.S. strategic forces.
Kathleen J. McInnis was the original author of this product.
U.S. Special Operations Command develops and
Nathan J. Lucas, Coordinator, Section Research Manager
employs fully capable Special Operations Forces to
conduct global special operations and activities as part
IF10543
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Defense Primer: The Department of Defense
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https://crsreports.congress.gov | IF10543 · VERSION 9 · UPDATED