https://crsreports.congress.gov
Updated November 8, 2021
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, comprising over 3
million employees stationed across the United States and
around the globe.
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, which helps
the Secretary plan, advise, and carry out the nation’s
security policies as directed by both the Secretary of
Defense and the President.
The Joint Chiefs of Staff and Joint Staff, which
collectively, through the Chairman of the Joint Chiefs of
Staff, provides advice to the President, the National
Security Council, the Homeland Security Council, and
the Secretary of Defense on military matters.
The Military Departments, which train and equip their
personnel to perform warfighting, peacekeeping and
humanitarian/disaster assistance tasks.
The Unified Combatant Commands, which deploy
troops and exercise military power on behalf of the
President and the Secretary of Defense with the advice
of the Joint Chiefs of Staff.
The Defense Agencies, sometimes called “DOD’s
fourth estate,” which provide or manage specific
capabilities for the department, such as logistics or
security cooperation.
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
the Department of Defense. 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 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 five military service chiefs
(Army, Navy, Air Force, Marine Corps and National Guard
Bureau), 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, the Homeland Security Council,
and the Secretary of Defense. According to Title 10, U.S.
Code, §151, the Chairman is the principal military advisor
to the President. Chairmen therefore have statutory
responsibility to present their 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 Chairman plans, coordinates, and oversees
military operations involving U.S. forces, neither the
Chairman 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 assist in
developing the unified strategic direction of the combatant
forces, their operation under unified command, and for 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, and Air Force. In practice, the Marines make up
about 20% of the number allocated to the Navy
(http://www.jcs.mil/About/).
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 Air Force. These departments are tasked with
training and equipping military forces to be utilized by the
combatant commands; departments are therefore
responsible for DOD’s “supply” of military forces and
equipment. Each department is led by a civilian service
secretary and supported by a service chief. A service chief
is a senior military officer designated as the principal
