https://crsreports.congress.gov
Updated January 26, 2022
Defense Primer: Legal Authorities for the Use of Military Forces
By the Framers’ apparent design, to keep the nation’s
“purse” and the “sword” in separate hands and in other
ways hinder the nation’s embroilment in unnecessary wars,
the Constitution divides war powers between Congress and
the President. Congress is empowered to declare war,
provide for and regulate the Armed Forces, and issue letters
of marque and reprisal, as well as to call forth the militia to
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
they conduct hostilities, put down insurrections, or execute
the law when constitutionally authorized to do so.
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
attack is the subject of long-standing debate. At the same
time, efforts in Congress to exercise its constitutional war
powers in some way that is perceived to constrain military
operations have met with objections that the constitutional
separation of powers is imperiled.
Overview
Congress has enacted 11 separate formal declarations of
war against foreign nations in five different wars, each time
preceded by a presidential request either in writing or in
person before a joint session of Congress.
Congress has also enacted authorizations for the use of
force rather than formal declarations of war. Such measures
have generally authorized military force against either a
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
than what he requested. Congress has also authorized the
President to use the military forces or the militia
domestically to put down insurrections or execute civilian
law when certain criteria are met. As noted in CRS Report
R42659, The Posse Comitatus Act and Related Matters:
The Use of the Military to Execute Civilian Law and CRS
Report RL31133, Declarations of War and Authorizations
for the Use of Military Force: Historical Background and
Legal Implications, Congress has provided approximately
50 statutory authorizations to use the military forces for
foreign or domestic purposes—not including formal
declarations of war.
As for the use of such authority, CRS Report R42738,
Instances of Use of United States Armed Forces Abroad,
1798-2021 lists hundreds of instances of U.S. uses of force
abroad, observing they reflect varying degrees of intensity
and longevity. It notes that most major uses of military
force abroad—of the type that might be classified as wars
or armed conflicts under international law—historically had
been authorized by Congress. The end of World War II
appears to have heralded a change in this regard. For
example, President Truman sent troops to defend South
Korea in 1950 under his own authority and a U.N. Security
Council resolution, but without specific authority from
Congress.
War Powers Resolution
Concern that too much of the war powers had accreted to
the President while Congress’s own authority had eroded
led to the 1973 enactment of the War Powers Resolution
(WPR; P.L. 93-148) over President Nixon’s veto. The WPR
asserts that the President has the authority to commit U.S.
troops to hostilities in only three sets of circumstances.
The WPR also attempts to circumscribe implied sources of
authority.
Presidents have taken a broader view of the Commander-in-
Chief power to use military force abroad. They have
variously asserted as sources of authority United Nations or
NATO decisions involving military intervention,
appropriations measures, and other statutes that do not
specifically cite the WPR. Additionally, they have relied on
the Commander-in-Chief power itself and the President’s
foreign affairs authority under Article II of the Constitution.
