Updated January 4, 2021
Defense Primer: Legal Authorities for the Use of Military Forces
By the Framers’ apparent design, to keep the nation’s
War Powers Resolution
“purse” and the “sword” in separate hands and in other
Concern that too much of the war powers had accreted to
ways hinder the nation’s embroilment in unnecessary wars,
the President while Congress’s own authority had eroded
the Constitution divides war powers between Congress and
led to the 1973 enactment of the War Powers Resolution
the President. Congress is empowered to declare war,
(WPR; P.L. 93-148) over President Nixon’s veto. The WPR
provide for and regulate the Armed Forces, and issue letters
asserts that the President has the authority to commit U.S.
of marque and reprisal, as well as to call forth the militia to
troops to hostilities in only three sets of circumstances.
suppress an insurrection, repel an invasion, or “execute the
Laws of the Union.” The President, as the Commander in
Chief, has the responsibility to direct the Armed Forces as
WPR Section 2(c) provides that the President’s
they conduct hostilities, put down insurrections, or execute
powers to introduce U.S. Armed Forces into
the law when constitutionally authorized to do so.
situations of hostilities or imminent hostilities are
exercised only pursuant to—
The extent to which the President has independent authority
under the Constitution, without explicit statutory support, to
(1) a declaration of war,
use the military for purposes other than to repel a sudden
attack is the subject of long-standing debate. At the same
(2) specific statutory authorization, or
time, efforts in Congress to exercise its constitutional war
powers in some way that is perceived to constrain military
(3) a national emergency created by attack upon the
operations have met with objections that the constitutional
United States, its territories or possessions, or its
separation of powers is imperiled.
Armed Forces.
Overview
Congress has enacted 11 separate formal declarations of
The WPR also attempts to circumscribe implied sources of
war against foreign nations in five different wars, each time
authority.
preceded by a presidential request either in writing or in
person before a joint session of Congress.
WPR Section 8 provides that the authority to
Congress has also enacted authorizations for the use of
introduce Armed Forces is not to be inferred from
force rather than formal declarations of war. Such measures
any provision of law or treaty unless such law, or
have generally authorized military force against either a
legislation implementing such treaty—
named country or unnamed hostile nations in a given
region. In most cases, the President has requested the
(a) specifical y authorizes the introduction of Armed
authority; Congress has sometimes given the President less
Forces into hostilities or potential hostilities, and
than what he requested. Congress has also authorized the
President to use the military forces or the militia
(b) states that it is intended to constitute specific
domestically to put down insurrections or execute civilian
statutory authorization within the meaning of the
law when certain criteria are met. As noted in CRS Reports
WPR.
R42659 and RL31133, Congress has provided
approximately 50 statutory authorizations to use the
military forces for foreign or domestic purposes—not
including formal declarations of war.
Presidents have taken a broader view of the Commander-in-
Chief power to use military force abroad. They have
As for the use of such authority, CRS Report R42731 lists
variously asserted as sources of authority United Nations or
hundreds of instances of U.S. uses of force abroad,
NATO decisions involving military intervention,
observing they reflect varying degrees of intensity and
appropriations measures, and other statutes that do not
longevity. It notes that most major uses of military force
specifically cite the WPR. Additionally, they have relied on
abroad—of the type that might be classified as wars or
the Commander-in-Chief power itself and the President’s
armed conflicts under international law—historically had
foreign affairs authority under Article II of the Constitution.
been authorized by Congress. The end of World War II
appears to have heralded a change in this regard. For
The executive branch has also occasionally attached
example, President Truman sent troops to defend South
significance to the failure of Congress to pass measures
Korea in 1950 under his own authority and a UN Security
introduced to prevent or end military operations overseas. It
Council resolution, but without specific authority from
has also interpreted some military uses of force to fall
Congress.
below the threshold of “hostilities” within the meaning of
the WPR.
https://crsreports.congress.gov
Defense Primer: Legal Authorities for the Use of Military Forces
Beginning in 2018, Congress has required the President to
report on the legal and policy frameworks for the United
Whoever, except in cases and under circumstances
States’ use of military force and related national security
expressly authorized by the Constitution or Act of
operations.
Congress, wil ful y uses any part of the Army or the
Air Force as a posse comitatus or otherwise to
Use of Military Forces to Execute
execute the laws shal be fined under this title or
Civilian Law
imprisoned not more than two years, or both.
Under the Constitution, states retain the primary
responsibility and authority to provide for civil order and
18 U.S.C. § 1385
the protection of their citizens’ lives and property.
However, the Constitution provides that the federal
government is responsible for protecting the states against
invasion and insurrection, and, if the state legislature (or the
The express statutory exceptions include legislation that
governor, if the legislature cannot be convened) requests it,
allows the President to use military force to suppress
protection against “domestic Violence.” While Congress is
insurrection or to enforce federal authority, and laws that
also empowered to authorize the militia to be called forth to
permit the Department of Defense to provide federal, state,
execute federal law, historical precedent s uggests that such
and local police with information, equipment, and
use was meant to be rare.
personnel.
The Insurrection Act
Case law indicates that “execution of the law” in violation
Soon after Congress was first assembled under the
of the PCA occurs (1) when civilian law enforcement
Constitution, it authorized the President to call out the
officials make “direct active use” of military investigators;
militia, initially to protect the frontier against “hostile
(2) when the use of the military “pervades the activities” of
incursions of the Indians,” and subsequently in cases of
the civilian officials; or (3) when the military is used to
invasion, insurrection, or obstruction of the laws.
subject “citizens to the exercise of military power which
Insurrections against state governments could be put down
was regulatory, prescriptive, or compulsory in nature.”
under the act only if the state legislature applied for such
However, the PCA is not violated when the Armed Forces
assistance. These provisions were quickly extended to alow
conduct activities for a military purpose. Additionally, the
for the employment of the Armed Forces in domestic
PCA does not apply to the National Guard, unless it is
circumstances, where the law already provided the militia
employed in federal service.
could be employed. After the Civil War, Congress added a
new provision for the use of federal military forces to
Relevant Statutes
protect civil rights.
50 U.S.C. §§ 1541-1549
The Insurrection Act has been invoked on dozens of
10 U.S.C. §§ 251-255 and §§ 271-284
occasions through U.S. history, although its use since the
18 U.S.C. § 1385
end of the 1960s civil rights disturbances has become
exceedingly rare. Its last invocation appears to have
occurred in 1992, when the acquittal of police officers on
charges of beating motorist Rodney King sparked rioting in
CRS Products
Los Angeles. Congress amended the statute in 2006 after
Hurricane Katrina raised concerns that the statutory
CRS Report RL31133, Declarations of War and Authorizations
requirements impeded the military’s ability to render
for the Use of Military Force: Historical Background and Legal
effective assistance amid the perceived breakdown of civil
Implications, by Jennifer K. Elsea and Matthew C. Weed
law and order, but repealed that amendment the following
CRS Report R42699, The War Powers Resolution: Concepts and
year after state governors objected to it.
Practice, by Matthew C. Weed
CRS Report R42738, Instances of Use of United States Armed
The Posse Comitatus Act
Forces Abroad, 1798-2020, by Barbara Salazar Torreon and
The Posse Comitatus Act (PCA) outlaws the willful use of
Sofia Plagakis
any part of the Army or Air Force to execute the law unless
CRS Report R41989, Congressional Authority to Limit Military
expressly authorized by the Constitution or an act of
Operations, by Jennifer K. Elsea, Michael John Garcia, and
Congress. The Navy and Marine Corps operate under
Thomas J. Nicola
similar restrictions pursuant to regulations.
CRS Report R42659, The Posse Comitatus Act and Related
Matters: The Use of the Military to Execute Civilian Law, by
Jennifer K. Elsea
Jennifer K. Elsea, Legislative Attorney
IF10539
https://crsreports.congress.gov
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 6 · UPDATED