Updated November 8, 2021
Defense Primer: The Department of Defense
The Department of Defense (DOD) was established after
capabilities for the department, such as logistics or
World War II through the 1947 National Security Act. At
security cooperation.
the time, some, including President Truman, took the view
that the different components of the U.S. military had been
The Office of the Secretary of Defense (OSD)
insufficiently integrated to wage World War II effectively.
Title 10 U.S. Code Section 113 specifies that the Secretary
The intention of the 1947 Act was therefore to create, for
of Defense exercises “authority, direction and control” over
the first time, an integrated institution that combined the
the Department of Defense. The Office of the Secretary of
Departments of War and Navy, and to establish a policy
Defense (OSD) assists the Secretary of Defense in
architecture for overseeing the newly reorganized military
exercising such authority over DOD. They do so in a
apparatus. Over time, DOD has grown into one of the
variety of areas, including policy development, planning,
largest bureaucracies in the world, comprising over 3
resource management, fiscal management, and program
million employees stationed across the United States and
evaluation. OSD also helps provide civilian oversight of the
around the globe.
military services and combatant commands to ensure that
the Secretary and the President’s defense objectives are
met.
“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
The Joint Chiefs of Staff (JCS)
The Joint Chiefs of Staff is the preeminent military
air forces at home as well as in other parts of the
advisory body in U.S. national security establishment. Its
world where our Armed Forces are serving. We did
membership consists of the five military service chiefs
not have that kind of direction when we were
(Army, Navy, Air Force, Marine Corps and National Guard
attacked four years ago—and we certainly paid a high
Bureau), the Chairman of the Joint Chiefs of Staff (CJCS),
price for not having it.”
and the Vice Chairman of the Joint Chiefs of Staff
President Harry S. Truman, “Message to Congress,”
(VCJCS). The JCS regularly convenes to formulate and
December 19, 1945.
provide its best military advice to the President, the
National Security Council, the Homeland Security Council,
and the Secretary of Defense. According to Title 10, U.S.
DOD’s purpose today is to provide the President with the
Code, §151, the Chairman is the principal military advisor
military forces needed to deter war and to protect the
to the President. Chairmen therefore have statutory
security of the country. It does so through five primary sets
responsibility to present their counsel—as well as any
of institutions, each representing thousands of people and
dissenting views from other members of the JCS—to senior
often hundreds of specific offices:
leaders in the U.S. national security establishment. Of note,
although the Chairman plans, coordinates, and oversees
The Office of the Secretary of Defense, which helps
military operations involving U.S. forces, neither the
the Secretary plan, advise, and carry out the nation’s
Chairman nor the JCS has a formal role in the execution of
security policies as directed by both the Secretary of
military operations—a role instead assigned to the unified
Defense and the President.
combatant commanders.
The Joint Chiefs of Staff and Joint Staff, which
The CJCS is supported by the Joint Staff, which assist in
collectively, through the Chairman of the Joint Chiefs of
developing the unified strategic direction of the combatant
Staff, provides advice to the President, the National
forces, their operation under unified command, and for their
Security Council, the Homeland Security Council, and
integration into an efficient team of land, naval, and air
the Secretary of Defense on military matters.
forces. The “Joint Staff” is composed of approximately
equal numbers of officers from the Army, Navy, Marine
The Military Departments, which train and equip their
Corps, and Air Force. In practice, the Marines make up
personnel to perform warfighting, peacekeeping and
about 20% of the number allocated to the Navy
humanitarian/disaster assistance tasks.
(http://www.jcs.mil/About/).
The Unified Combatant Commands, which deploy
The Military Departments
troops and exercise military power on behalf of the
There are three military departments: the Army, Navy and
President and the Secretary of Defense with the advice
Air Force. The Marine Corps, mainly an amphibious force,
of the Joint Chiefs of Staff.
is part of the Department of the Navy. The Space Force is
part of the Air Force. These departments are tasked with
The Defense Agencies, sometimes called “DOD’s
training and equipping military forces to be utilized by the
fourth estate,” which provide or manage specific
combatant commands; departments are therefore
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Defense Primer: The Department of Defense
responsible for DOD’s “supply” of military forces and
 U.S. Strategic Command, which is responsible for
equipment. Each department is led by a civilian service
controlling space, deterring attacks on the United States
secretary and supported by a service chief. A service chief
and its allies, launching and operating satellite systems,
is a senior military officer designated as the principal
and directing the use of U.S. strategic forces;
military advisor to a departmental secretary for matters
relating to a specific armed service.
 U.S. Special Operations Command, which provides
counter-paramilitary, counter-narcotics, guerilla,
In terms of overall roles and responsibilities, The Army
psychological warfare, civil education, and insurgency
defends the land mass of the United States, its territories,
capabilities; and
commonwealths, and possessions; it operates in more than
50 countries. The Navy maintains, trains, and equips
 U.S. Transportation Command (USTRANSCOM),
combat-ready maritime forces capable of winning wars,
which provides air, land, and sea transportation to
deterring aggression, and maintaining freedom of the seas.
different components of the Department of Defense.
The U.S. Marine Corps maintains ready expeditionary
forces, sea-based and integrated air-ground units for
 U.S. Cyber Command (USCYBERCOM) directs,
contingency and combat operations, and the means to
synchronizes, and coordinates cyberspace planning and
stabilize or contain international disturbance. The Air Force
operations to defend and advance national interests in
provides a rapid, flexible, and, when necessary, lethal air
collaboration with domestic and international partners.
and space capability that can deliver forces anywhere in the
world in less than 48 hours.
Defense Agencies
Title 10, U.S.C., §191 grants the Secretary of Defense
Unified Combatant Commands
authority to establish agencies that provide for the
The Unified Combatant Commands, or “COCOMs,” are the
performance of a supply or service activity that is common
principal mechanism through which the Department of
to more than one military department, in instances where
Defense conducts its global operations. The COCOMs
doing so is deemed more effective, economical, or efficient
represent the “demand” side of DOD, as it is primarily the
than existing structures. Organizations established under
COCOM’s operational needs that drive the development of
this authority are referred to as either Defense Agencies or a
military requirements across the department.
Department of Defense Field Activity. According to DOD’s
Chief Management Officer, there are 20 Defense Agencies
There are seven regionally focused COCOMS:
(9 of which are also designated Combat Support Agencies

pursuant to Title 10, U.SC., §193, meaning that they are
U.S. Africa Command, responsible for sub-Saharan
jointly overseen by the Chairman of the Joint Chiefs of
Africa;
Staff), and 8 DOD Field Activities.
 U.S. European Command, responsible for all of Europe,
DOD Reform
large portions of Central Asia, parts of the Middle East,
Managing an organization as large and complex as DOD
and the Arctic and Atlantic Oceans;
presents a unique challenge to its senior leaders. Concerns

about the department’s efficiency, effectiveness, or both
U.S. Central Command, responsible for most of the
have driven reform initiatives since the establishment of
Middle East, parts of Northern Africa and west Asia,
DOD itself. One particular tension that was built into
and part of the Indian Ocean;
Truman’s initial DOD design is that between the military

services on the one hand, which strive to protect and
U.S. Northern Command, responsible for defense of the
advance their institutional ethos and capabilities, and joint
continental United States and coordination of security
or defense-wide institutions on the other, which seek to
and military relationships with Canada and Mexico;
advance a greater degree of integration between DOD’s

constituent parts. In the early 1980s, congressional concerns
U.S. Southern Command, responsible for Central
that the services had undue—and unhelpful—influence in
America, South America, and the Caribbean; and
the conduct of military operations led to the 1986

Goldwater-Nichols Defense Reform Act. Thirty years later,
U.S. Indo-Pacific Command, responsible for the Pacific
the FY2017 National Defense Authorization Act (S. 2943)
Ocean, Southwest Asia, Australia, South Asia, and parts
also sought to reform the way DOD is organized, with the
of the Indian Ocean. It shares responsibility for Alaska
overall aim of improving the department’s agility and
with U.S. Northern Command.
strategic integration of its global assets.
 U.S. Space Command (USSPACECOM), established in
CRS Products
2019, which deters aggression and conflict, defends U.S.
and allied freedom of action, delivers space combat
CRS Report R44474, Goldwater-Nichols at 30: Defense Reform
power for the Joint/Combined force, and develops joint
and Issues for Congress, by Kathleen J. McInnis
warfighters to advance U.S. and allied interests in, from,
and through the space domain.

There are also four “functional” COCOMs:
Kathleen J. McInnis, Analyst in International Security
IF10543
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Defense Primer: The Department of Defense


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