Order Code RS21314
Updated September 23, 2002
CRS Report for Congress
Received through the CRS Web
International Law and the Preemptive Use of
Force Against Iraq
David M. Ackerman
Legislative Attorney
American Law Division
Summary
In his speech to the United Nations on September 12, 2002, President Bush
described the regime of Saddam Hussein in Iraq as “a grave and gathering danger,”
detailed that regime’s persistent efforts to acquire weapons of mass destruction and its
persistent defiance of numerous Security Council resolutions requiring Iraq to disarm,
and raised the specter of an “outlaw regime” providing such weapons to terrorists. The
President left little doubt that, with or without UN support, the United States would act
to force Iraq to disarm and otherwise abide by its past commitments, and that military
force might well be used to accomplish that objective. One issue raised by that policy
is the legality of the preemptive use of force under international law. This report
examines that issue as developed in customary international law and under the United
Nations Charter. It will be updated as events warrant. (For historical information on the
preemptive use of force by the U.S., see CRS Report RS21311, U.S. Use of Preemptive
Use of Force.
Preemptive Military Attacks Under Customary International Law
Until recent decades customary international law deemed the right to use force and
even to go to war to be an essential attribute of every state. As one scholar summarized:
It always lies within the power of a State to endeavor to obtain redress for wrongs, or
to gain political or other advantages over another, not merely by the employment of
force, but also by direct recourse to war.1
Within that framework customary international law also consistently recognized self-
defense as a legitimate basis for the use of force:
An act of self-defense is that form of self-protection which is directed against an
aggressor or contemplated aggressor. No act can be so described which is not
1 Hyde, Charles Cheney, International Law Chiefly As Interpreted and Applied by the United
States, Vol. 3 (1945), at 1686.
Congressional Research Service ˜ The Library of Congress

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occasioned by attack or fear of attack. When acts of self-preservation on the part of
a State are strictly acts of self-defense, they are permitted by the law of nations, and
are justified on principle, even though they may conflict with the ... rights of other
states.2
Moreover, the recognized right of a state to use force for purposes of self-defense
traditionally included the preemptive use of force, i.e., the use of force in anticipation of
an attack. Hugo Grotius, the father of international law, stated in the seventeenth century
that “[i]t be lawful to kill him who is preparing to kill.”3 Emmerich de Vattel a century
later similarly asserted:
The safest plan is to prevent evil, where that is possible. A Nation has the right to
resist the injury another seeks to inflict upon it, and to use force ... against the
aggressor. It may even anticipate the other’s design, being careful, however, not to
act upon vague and doubtful suspicions, lest it should run the risk of becoming itself
the aggressor.4
The classic formulation of the right of preemptive attack was given by Secretary of
State Daniel Webster in connection with the famous Caroline incident. In 1837 British
troops under the cover of night attacked and sank an American ship, the Caroline, in U.S.
waters because the ship was being used to provide supplies to insurrectionists against
British rule in Canada headquartered on an island on the Canadian side of the Niagara
River. The U.S. immediately protested this “extraordinary outrage” and demanded an
apology and reparations. The dispute dragged on for several years before the British
conceded that they ought to have immediately offered “some explanation and apology.”
But in the course of the diplomatic exchanges Secretary of State Daniel Webster
articulated the two conditions essential to the legitimacy of the preemptive use of force
under customary international law. In one note he asserted that an intrusion into the
territory of another state can be justified as an act of self-defense only in those “cases in
which the necessity of that self-defense is instant, overwhelming, and leaving no choice
of means and no moment for deliberation.”5 In another note he asserted that the force
used in such circumstances has to be proportional to the threat:
It will be for [Her Majesty’s Government] to show, also, that the local authorities of
Canada, even supposing the necessity of the moment authorized them to enter the
territories of the United States at all, did nothing unreasonable or excessive; since the
act, justified by the necessity of self-defence, must be limited by that necessity, and
kept clearly within it.6
2 Id. Vol. 1, at 237.
3 Grotius, Hugo, The Law of War and Peace, at 1625.
4 de Vattel, Emmerich, The Law of Nations, Vol. IV, at 3.
5 Letter from Secretary of State Daniel Webster to Lord Ashburton of August 6, 1842, set forth
in Moore, John Bassett, A Digest of International Law, Vol. II (1906), at 412.
6 Letter from Mr. Webster to Mr. Fox of April 24, 1841, 29 British and Foreign State Papers
1129, 1138 (1857), quoted in Damrosch, Lori, International Law: Cases and Materials (2001),
at 923.

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Both elements – necessity and proportionality – have been deemed essential to legitimate
the preemptive use of force in customary international law.7
Effect of the United Nations Charter
However, with the founding of the United Nations, the legitimacy of the use of force
by individual states under international law has been substantially narrowed. The Charter
of the UN states in its Preamble that the UN is established “to save succeeding
generations from the scourge of war”; and its substantive provisions obligate the Member
States of the UN to “settle their international disputes by peaceful means” (Article 2(3))
and 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 manner inconsistent
with the Purposes of the United Nations” (Article 2(4)). In place of the traditional right
of states to use force, the Charter creates a system of collective security in which the
Security Council is authorized to “determine the existence of any threat to the peace,
breach of the peace, or act of aggression” and to “decide what measures shall be taken ...
to maintain international peace and security” (Article 39).
Although nominally outlawing most uses of force in international relations by
individual States, the UN Charter does recognize a right of nations to use force for the
purpose of self-defense. Article 51 of the Charter provides:
Nothing 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.8
The exact scope of this right of self-defense, however, has been the subject of ongoing
debate. Read literally, Article 51's articulation of the right seems to preclude the
preemptive use of force by individual states or groupings of states and to reserve such
uses of force exclusively to the Security Council. Measures in self-defense, in this
understanding, are legitimate only after an armed attack has already occurred.9
7 In its advisory opinion on the Legality of the Threat or Use of Nuclear Weapons, the
International Court of Justice stated that “[t]he submission of the exercise of the right of self-
defence to the conditions of necessity and proportionality is a rule of customary international
law.” 1996 I.C.J. Reports para. 41.
8 United Nations Charter, Article 51.
9 This reading of Article 51 finds support in the decision of the International Court of Justice in
Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States of
America), 1986 I.C.J. Reports p. 14. The Court decided the case on the basis of customary
international law rather than Articles 2(4) and 51 of the UN Charter but found the latter to
“correspond, in essentials, to those found in customary international law.” The gravamen of the
Court’s ruling was that in customary international law as well as Article 51, the use of force in
self-defense is justified only in response to an armed attack:
... [F]or one State to use force against another ... is regarded as lawful, by way of
exception, only when the wrongful act provoking the response was an armed attack
.... In the view of the Court, under international law in force today – whether
(continued...)

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Others contend that Article 51 should not be construed so narrowly and that “it
would be a travesty of the purposes of the Charter to compel a defending state to allow
its assailant to deliver the first, and perhaps fatal, blow ....”10 To read Article 51 literally,
it is said, “is to protect the aggressor’s right to the first strike.”11 Consequently, to avoid
this result, some assert that Article 51 recognizes the “inherent right of individual or
collective self-defence” as it developed in customary international law prior to adoption
of the Charter and preserves it intact. The reference to that right not being impaired “if
an armed attack occurs against a Member of the United Nations,” it is said, merely
emphasizes one important situation where that right may be exercised but does not
exclude or exhaust other possibilities.12
In further support of this view, it is argued that the literal construction of Article 51
simply ignores the reality that the Cold War and other political considerations have often
paralyzed the Security Council and that, in practice, states have continued to use force
preemptively at times in the UN era and the international community has continued to
evaluate the legitimacy of those uses by the traditional constraints of necessity and
proportionality. Examples used to illustrate these contentions have included the
following:
! In 1962 President Kennedy, in response to photographic evidence that the
Soviet Union was installing medium range missiles in Cuba capable of
hitting the United State, imposed a naval “quarantine” on Cuba in order
“to interdict ... the delivery of offensive weapons and associated
material.”13 Although President Kennedy said that the purpose of the
quarantine was “to defend the security of the United States,” the U.S. did
not rely on the legal concept of self-defense either as articulated in
Article 51 or otherwise as a justification for its actions. Abram Chayes,
9 (...continued)
customary international law or that of the United Nations system – States do not have
a right of “collective” armed response to acts which do not constitute an “armed
attack.”
Id. para. 211.
10 Statement by Sir Humphrey Waldock, quoted in Roberts, Guy, “The Counterproliferation Self-
Help Paradigm: A Legal Regime for Enforcing the Norm Prohibiting the Proliferation of
Weapons of Mass Destruction,” 27 Denver Journal of International Law and Policy 483, 513
(1999).
11 Id.
12 Simma, Bruno, ed., The Charter of the United Nations: A Commentary (1994), at 51. This
contention finds some support in the advisory opinion of the International Court of Justice in
Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. Reports para. 96-97. In passing
on the question of whether it might ever be legal for a nation to use nuclear weapons, the Court
refused to construe Article 51 or customary international law to preclude “the use of nuclear
weapons by a State in an extreme circumstance of self-defence, in which its very survival would
be at stake.” The Court’s decision did not specifically deal with the question of preemptive
attack. But it seems to give support to an expansive understanding of what might be permissible
in instances of extreme necessity.
13 Proclamation 3504, 27 Fed. Reg. 10401 (October 25, 1962).

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the Legal Adviser to the State Department at that time, later explained the
decision not to rely on that justification as follows:
In retrospect ... I think the central difficulty with the Article 51 argument
was that it seemed to trivialize the whole effort at legal justification. No
doubt the phrase “armed attack” must be construed broadly enough to
permit some anticipatory response. But it is a very different matter to
expand it to include threatening deployments or demonstrations that do not
have imminent attack as their purpose or probable outcome. To accept that
reading is to make the occasion for forceful response essentially a question
for unilateral national decision that would not only be formally
unreviewable, but not subject to intelligent criticism, either .... Whenever
a nation believed that interests, which in the heat and pressure of a crisis
it is prepared to characterize as vital, were threatened, its use of force in
response would become permissible .... In this sense, I believe that an
Article 51 defence would have signalled that the United States did not take
the legal issues involved very seriously, that in its view the situation was
to be governed by national discretion, not international law.14
! In 1967 Israel launched a preemptive attack on Egypt and other Arab
states after President Nasser had moved his army across the Sinai toward
Israel, forced the UN to withdraw its peacekeeping force from the Sinai
border, and closed the port of Aqaba to Israeli shipping, and after Syria,
Iraq, Jordan, and Saudi Arabia all began moving troops to the Israeli
borders. In six days it routed Egypt and its Arab allies and had occupied
the Sinai Peninsula, the West Bank, and the Gaza Strip. Israel claimed
its attack was defensive in nature and necessary to forestall an Arab
invasion. Both the Security Council and the General Assembly rejected
proposals to condemn Israel for its “aggressive” actions.15

! On June 7, 1981, Israel bombed and destroyed a nuclear reactor under
construction in Iraq. Asserting that Iraq considered itself to be in a state
of war with Israel, that it had participated in the three wars with Israel in
1948, 1967, and 1973, that it continued to deny that Israel has a right to
exist, and that its nuclear program was for the purpose of developing
weapons capable of destroying Israel, Israel claimed that “in removing
this terrible nuclear threat to its existence, Israel was only exercising its
legitimate right of self-defense within the meaning of this term in
international law and as preserved also under the United Nations
Charter.”16 Nonetheless, the Security Council unanimously
“condemn[ed] the military attack by Israel in clear violation of the
14 See Chayes, A., The Cuban Missile Crisis (1974), at 63-64, quoted in Carter, Barry, and
Trimble, Phillip, International Law (1999), at 1241-42.
15 The Security Council, instead, adopted Resolution 242 calling on Israel to withdraw from the
territories and for the termination of all claims or states of belligerency and the acknowledgment
of the territorial integrity and the right of every State in the region to live in peace.
16 20 ILM 996 (July, 1981) (excerpts from Security Council debate).

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Charter of the United Nations and the norms of international conduct”
and urged the payment of “appropriate redress.”17
Current Situation
Thus, in both theory and practice the preemptive use of force appears to have a home
in current international law; but its boundaries are not wholly determinate. Its clearest
legal foundation is in Chapter VII of the UN Charter. Under Article 39 the Security
Council has the authority to determine the existence not only of breaches of the peace or
acts of aggression that have already occurred but also of threats to the peace; and under
Article 42 it has the authority to “take such action by air, sea, or land forces as may be
necessary to maintain or restore international peace and security.” These authorities
clearly seem to encompass the possibility of the preemptive use of force. As a
consequence, the preemptive use of force by the United States against Iraq or any other
sovereign nation pursuant to an appropriate authorization by the Security Council would
seem to be consonant with international law. Less clear is whether international law
currently allows the preemptive use of force by a nation or group of nations without
Security Council authorization. That would seem to be permissible only if Article 51 is
not read literally but expansively to preserve as lawful the use of force in self-defense as
traditionally allowed in customary international law. As noted, the construction of Article
51 remains a matter of debate. But so construed, Article 51 would not preclude the
preemptive use of force by the U.S. against Iraq or other sovereign nations. To be lawful,
however, such uses of force would need to meet the traditional requirements of necessity
and proportionality.
If customary international law governing the preemptive use of force does remain
valid, a primary difficulty still remains of determining what situations meet the test of
necessity. As illustrated in the examples listed above, that requirement is most easily met
when an armed attack is clearly imminent, as in the case of the Arab-Israeli War of 1967.
But beyond such obvious situations, as Abram Chayes argued, the judgment of necessity
becomes increasingly subjective; and there is at present no consensus either in theory or
practice about whether the possession or development of weapons of mass destruction by
a rogue state justifies the preemptive use of force. Most analysts recognize that if
overwhelmingly lethal weaponry is possessed by a nation willing to use that weaponry
directly or through surrogates, some kind of anticipatory self-defense may be a matter of
national survival; and many contend that international law ought, if it does not already do
so, to allow for the preemptive use of force in that situation. But many states and analysts
are decidedly reluctant to legitimate the preemptive use of force even in that situation on
the grounds the justification can easily be abused. Moreover, it remains a fact that the
international community judged Israel’s destruction of Iraq’s nuclear reactor site in 1981
to be an aggressive act rather than an act of self-defense; and there has not been any
further refinement of the characteristics that might make analogous situations meet the
test of necessity. Given this fluidity, the legality under international law of a preemptive
attack on Iraq or another sovereign nation by the U.S. in the current situation, if done
apart from authorization by the Security Council, may not be able to be judged with any
certainty at the outset. Moreover, it may ultimately have the effect of shaping what is
deemed to be a lawful preemptive use of force.
17 Id. at 993 (S/RES/487 adopted on June 19, 1981).

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