https://crsreports.congress.gov
Updated November 30, 2022
Defense Primer: The Department of Defense
The Department of Defense (DOD) was established after
World War II through the 1947 National Security Act. At
the time, some, including President Truman, took the view
that the different components of the U.S. military had been
insufficiently integrated to wage World War II effectively.
The intention of the 1947 Act was therefore to create, for
the first time, an integrated institution that combined the
Departments of War and Navy, and to establish a policy
architecture for overseeing the newly reorganized military
apparatus. Over time, DOD has grown into one of the
largest bureaucracies in the world, currently comprising
over 2.9 million service members and civilians stationed in
4,800 sites across the United States and around the globe.
Congress, in its constitutional role, legislates both
authorization and appropriations bills, as well as conducting
oversight on all of DOD. The Senate confirms multiple
military officers and certain DOD civilian officials.
DOD’s purpose today is to provide the President with the
military forces needed to deter war and to protect the
security of the country. It does so through five primary sets
of institutions, each representing thousands of people and
often hundreds of specific offices:
The Office of the Secretary of Defense;
The Joint Chiefs of Staff and Joint Staff;
The Military Departments;
The Unified Combatant Commands; and
The Defense Agencies.
These institutions are described below.
The Office of the Secretary of Defense (OSD)
Title 10 U.S. Code Section 113 specifies that the Secretary
of Defense exercises “authority, direction and control” over
DOD. The Office of the Secretary of Defense (OSD) assists
the Secretary of Defense in exercising such authority over
DOD. They do in a variety of areas, including policy
development, planning, resource management, fiscal
management, and program evaluation. OSD also helps
provide civilian oversight of the military services and
combatant commands to ensure that the Secretary’s and the
President’s defense objectives are met.
The Joint Chiefs of Staff (JCS)
The Joint Chiefs of Staff is the preeminent military
advisory body in U.S. national security 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 its best military advice to the President, the
National Security Council, and the Secretary of Defense.
According to Title 10, U.S.C., §151, the Chairman is the
principal military advisor to the President. The CJCS
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
security establishment. Of note, although the CJCS plans,
coordinates, and oversees military operations involving
U.S. forces, neither the CJCS nor the JCS has a formal role
in the execution of military operations—a role instead
assigned to the unified combatant commanders.
The CJCS is supported by the Joint Staff, which assists in
developing the unified strategic direction of the combatant
forces, their operation under unified command, and their
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,
Air Force, and Space Force, as well as Department of
Defense civilians. In practice, the Marines make up about
20% of the number allocated to the Navy.
The Military Departments
There are three military departments: the Army, Navy and
Air Force. The Marine Corps, mainly an amphibious force,
is part of the Department of the Navy. The Space Force is
part of the Department of the Air Force. These departments
are responsible for training and equipping the military
forces utilized by the combatant commands; departments
therefore produce DOD’s “supply” of military forces and
equipment. Each department is led by a civilian service
secretary and supported by a service chief, both confirmed
by the Senate. A service chief is a senior military officer
designated as the principal military advisor to a
departmental secretary for matters relating to a specific
armed service.
In terms of overall roles and responsibilities, The Army
trains and equips forces to provide ready, prompt, and
sustained land dominance across the full spectrum of
conflict as part of the joint force. The Navy maintains,
trains, and equips combat-ready maritime forces capable of