Updated January 26, 2022
Defense Primer: Legal Authorities for the Use of Military Forces
By the Framers’ apparent design, to keep the nation’s
appears to have heralded a change in this regard. For
“purse” and the “sword” in separate hands and in other
example, President Truman sent troops to defend South
ways hinder the nation’s embroilment in unnecessary wars,
Korea in 1950 under his own authority and a U.N. Security
the Constitution divides war powers between Congress and
Council resolution, but without specific authority from
the President. Congress is empowered to declare war,
Congress.
provide for and regulate the Armed Forces, and issue letters
of marque and reprisal, as well as to call forth the militia to
War Powers Resolution
suppress an insurrection, repel an invasion, or “execute the
Concern that too much of the war powers had accreted to
Laws of the Union.” The President, as the Commander in
the President while Congress’s own authority had eroded
Chief, has the responsibility to direct the Armed Forces as
led to the 1973 enactment of the War Powers Resolution
they conduct hostilities, put down insurrections, or execute
(WPR; P.L. 93-148) over President Nixon’s veto. The WPR
the law when constitutionally authorized to do so.
asserts that the President has the authority to commit U.S.
troops to hostilities in only three sets of circumstances.
The extent to which the President has independent authority
under the Constitution, without explicit statutory support, to
use the military for purposes other than to repel a sudden
WPR Section 2(c) provides that the President’s
attack is the subject of long-standing debate. At the same
powers to introduce U.S. Armed Forces into
time, efforts in Congress to exercise its constitutional war
situations of hostilities or imminent hostilities are
powers in some way that is perceived to constrain military
exercised only pursuant to—
operations have met with objections that the constitutional
separation of powers is imperiled.
(1) a declaration of war,
Overview
(2) specific statutory authorization, or
Congress has enacted 11 separate formal declarations of
war against foreign nations in five different wars, each time
(3) a national emergency created by attack upon the
preceded by a presidential request either in writing or in
United States, its territories or possessions, or its
person before a joint session of Congress.
Armed Forces.
Congress has also enacted authorizations for the use of
force rather than formal declarations of war. Such measures
The WPR also attempts to circumscribe implied sources of
have generally authorized military force against either a
authority.
named country or unnamed hostile nations in a given
region. In most cases, the President has requested the
authority; Congress has sometimes given the President less
WPR Section 8 provides that the authority to
than what he requested. Congress has also authorized the
introduce Armed Forces is not to be inferred from
President to use the military forces or the militia
any provision of law or treaty unless such law, or
domestically to put down insurrections or execute civilian
legislation implementing such treaty—
law when certain criteria are met. As noted in CRS Report
R42659, The Posse Comitatus Act and Related Matters:
(a) specifically authorizes the introduction of Armed
The Use of the Military to Execute Civilian Law and CRS
Forces into hostilities or potential hostilities, and
Report RL31133, Declarations of War and Authorizations
for the Use of Military Force: Historical Background and
(b) states that it is intended to constitute specific
Legal Implications, Congress has provided approximately
statutory authorization within the meaning of the
50 statutory authorizations to use the military forces for
WPR.
foreign or domestic purposes—not including formal
declarations of war.
As for the use of such authority, CRS Report R42738,
Presidents have taken a broader view of the Commander-in-
Instances of Use of United States Armed Forces Abroad,
Chief power to use military force abroad. They have
1798-2021 lists hundreds of instances of U.S. uses of force
variously asserted as sources of authority United Nations or
abroad, observing they reflect varying degrees of intensity
NATO decisions involving military intervention,
and longevity. It notes that most major uses of military
appropriations measures, and other statutes that do not
force abroad—of the type that might be classified as wars
specifically cite the WPR. Additionally, they have relied on
or armed conflicts under international law—historically had
the Commander-in-Chief power itself and the President’s
been authorized by Congress. The end of World War II
foreign affairs authority under Article II of the Constitution.
https://crsreports.congress.gov
Defense Primer: Legal Authorities for the Use of Military Forces
The executive branch has also occasionally attached
significance to the failure of Congress to pass measures
Whoever, except in cases and under circumstances
introduced to prevent or end military operations overseas. It
expressly authorized by the Constitution or Act of
has also interpreted some military uses of force to fall
Congress, willfully uses any part of the Army, Navy,
below the threshold of “hostilities” within the meaning of
Marine Corps, Air Force, and Space Force as a posse
the WPR.
comitatus or otherwise to execute the laws shall be
fined under this title or imprisoned not more than
Beginning in 2018, Congress has required the President to
two years, or both.
report on the legal and policy frameworks for the United
States’ use of military force and related national security
18 U.S.C. § 1385 (as amended)
operations.
Use of Military Forces to Execute
Civilian Law
The express statutory exceptions include legislation that
Under the Constitution, states retain the primary
allows the President to use military force to suppress
responsibility and authority to provide for civil order and
insurrection or to enforce federal authority, and laws that
the protection of their citizens’ lives and property. The
permit the Department of Defense to provide federal, state,
federal government is responsible for protecting the states
and local police with information, equipment, and
against invasion and insurrection, and, if the state
personnel.
legislature (or the governor, if the legislature cannot be
convened) requests it, protection against “domestic
Case law indicates that “execution of the law” in violation
Violence.” While Congress is also empowered to authorize
of the PCA occurs (1) when civilian law enforcement
the militia to be called forth to execute federal law,
officials make “direct active use” of military investigators;
historical precedent suggests that such use was meant to be
(2) when the use of the military “pervades the activities” of
rare.
the civilian officials; or (3) when the military is used to
subject “citizens to the exercise of military power which
The Insurrection Act
was regulatory, prescriptive, or compulsory in nature.” The
Soon after Congress was first assembled under the
PCA is not violated when the Armed Forces conduct
Constitution, it authorized the President to call out the
activities for a military purpose. Additionally, the PCA does
militia, initially to protect the frontier against “hostile
not apply to the National Guard, unless it is employed in
incursions of the Indians,” and subsequently in cases of
federal service.
invasion, insurrection, or obstruction of the laws.
Insurrections against state governments could be put down
Relevant Statutes
under the act only if the state legislature applied for such
assistance. These provisions were quickly extended to allow
50 U.S.C. §§ 1541-1549
for the employment of the Armed Forces in domestic
10 U.S.C. §§ 251-255 and §§ 271-284
circumstances, where the law already provided the militia
18 U.S.C. § 1385
could be employed. After the Civil War, Congress added a
new provision for the use of federal military forces to
protect civil rights.
CRS Products
The Insurrection Act has been invoked on dozens of
CRS Report RL31133, Declarations of War and Authorizations
occasions through U.S. history, although its use since the
for the Use of Military Force: Historical Background and Legal
end of the 1960s civil rights disturbances has become
Implications, by Jennifer K. Elsea and Matthew C. Weed
exceedingly rare. Its last invocation appears to have
occurred in 1992, when the acquittal of police officers on
CRS Report R42699, The War Powers Resolution: Concepts and
charges of beating motorist Rodney King sparked rioting in
Practice, by Matthew C. Weed
Los Angeles. Congress amended the statute in 2006 after
CRS Report R42738, Instances of Use of United States Armed
Hurricane Katrina raised concerns that the statutory
Forces Abroad, 1798-2021, by Barbara Salazar Torreon and
requirements impeded the military’s ability to render
Sofia Plagakis
effective assistance amid the perceived breakdown of civil
CRS Report R41989, Congressional Authority to Limit Military
law and order, but repealed that amendment the following
Operations, by Jennifer K. Elsea, Michael John Garcia, and
year after state governors objected to it.
Thomas J. Nicola
The Posse Comitatus Act
CRS Report R42659, The Posse Comitatus Act and Related
The Posse Comitatus Act (PCA) outlaws the willful use of
Matters: The Use of the Military to Execute Civilian Law, by
any part of the Armed Forces to execute the law unless
Jennifer K. Elsea
expressly authorized by the Constitution or an act of
Congress.
Jennifer K. Elsea, Legislative Attorney
IF10539
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Defense Primer: Legal Authorities for the Use of Military Forces
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff to
congressional committees and Members of Congress. It operates solely at the behest of and under the direction of Congress.
Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has
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https://crsreports.congress.gov | IF10539 · VERSION 9 · UPDATED