Response to Terrorism: Legal Aspects of the Use of Military Force
CRS Report for Congress
Received through the CRS Web
Response to Terrorism: Legal Aspects of the
Use of Military Force
David M. Ackerman
American Law Division
The terrorist attack of September 11, 2001, has precipitated widespread calls for
the use of military force in response. Under U.S. and international law a variety of legal
considerations attach to such use. This report briefly summarizes several salient aspects.
Legal considerations concerning the use of force under U.S. and international law
include the following:
(1) Acts of war. War has been defined as “a condition of armed hostility between
States”1 or, in Grotius’ terms, as the “state or condition of governments contending by
force.”2 An act of war, thus, involves the threat or use of force of some kind by one state
against another. But whether a particular threat or use of force constitutes an act of war
depends heavily on how the parties choose to characterize it. For the United States acts
of foreign governments that have been deemed to constitute acts of war have ranged from
the impressment of U.S. seamen into service in the British navy to failure to honor U.S.
neutrality in conflicts between other states to Japan’s attack on Pearl Harbor.
(2) Declaration of war. Article I, § 8, of the Constitution confers on Congress the
power to “declare War.” That power comprehends not only the enactment of formal
declarations of war but also the authorization of uses of military force which are not
intended to rise to the level of a war. Congress has enacted eleven formal declarations of
war relating to five different conflicts – the War of 1812, the Mexican-American War in
1846, the Spanish-American War in 1898, World War I, and World War II. It has also
enacted numerous authorizations for the use of military force that have not constituted
declarations of war, such as the Tonkin Gulf Resolution of 19643 and the 1991
1 Hyde, Charles Cheney, International Law Chiefly as Interpreted and Applied by the United
States, Vol. 3 (1945), at 1686.
2 Id. n. 1. Grotius was one of the original theorists of international law.
3 P.L. 88-408 (August 10, 1964).
Congressional Research Service ˜ The Library of Congress
“Authorization for Use of Military Force Against Iraq Resolution.”4 One domestic effect
of a declaration of war is that it brings into effect a number of statutes that confer
discretionary authority on the President.
(3) President’s authority to use military force. Article II, § 1, of the
Constitution vests the “executive Power” of the government in the President. Article II,
§ 2, states that the President “shall be Commander in Chief of the Army and Navy of the
United States, and of the Militia of the several States, when called into the actual Service
of the United States.” These clauses clearly empower the President to direct the conduct
of a war or other military engagement authorized by Congress. But it has often been
argued that these clauses, as well as the inherent authority that accrues to the President by
virtue of the existence of the United States as a sovereign nation, empower the President
to initiate the use of force even absent Congressional authorization. Others contend
otherwise. But however that debate is resolved in a given situation, virtually all
commentators concur that the President has the constitutional authority to defend the
United States from sudden attack even absent Congressional authorization. Whether that
authority is wholly defensive or can also involve the use of offensive force in a given
situation, however, has been the subject of dispute between Congress and the President.
(4) War Powers Resolution. To protect its constitutional role regarding the use
of military force, Congress in 1973 enacted the “War Powers Resolution” (WPR)5 over
President Nixon’s veto. That enactment specifically recognizes the President’s authority
to use military force in “a national emergency created by attack upon the United States,
its territories or possessions, or its armed forces.” But it requires the President to “consult
with Congress ... in every possible instance” prior to employing force and to submit a
report to Congress on any such instance. Most critically, it requires the President to
terminate U.S. involvement in hostilities within 60-90 days unless Congress has enacted
a declaration of war or a specific authorization for such involvement, has extended the 60-
day period by statute, or is unable to meet because of an armed attack on the United
States. Every President since Nixon has regarded the WPR as an unconstitutional
intrusion on his prerogatives regarding the use of force.
(5) United Nations Charter. The UN Charter was adopted in part “to save
succeeding generations from the scourge of war” (Preamble). As a consequence, it
requires its Members to “refrain in their international relations from the threat or use of
force against the territorial integrity or political independence of any State, or in any other
manner inconsistent with the Purposes of the United Nations” (Article 2(4)); and it
provides for collective action under the auspices of the Security Council to maintain or
restore international peace and security (Chapter VII). However, the Charter also
recognizes a right of self-defense. Article 51 provides that “[n]othing in the present
Charter shall impair the inherent right of individual or collective self-defence if an armed
attack occurs against a Member of the United Nations, until the Security Council has taken
measures necessary to maintain international peace and security ....”
4 P.L. 102-1 (Jan. 14, 1991).
5 P.L. 93-148 (Nov. 7, 1973); 50 U.S.C. 1541 et seq.