The use of force

AuthorJohn H. Currie, Craig Forcese, Joanna Harrington, Valerie Oosterveld
In this chapter, we consider the extent to which international law permits s tates to use
military force against one another (and possibly against non-state actors). We discuss this
as a matter of international law enforcement because the circumstances in which states
may still lawfully use such force have effec tively been narrowed to those in which the tar-
get state is in breach of certain of its international legal obligations.
This is not to suggest, however, that the use of force is generally available as a re-
sponse to the violation by a state of any of its international legal obligations. As previously
noted in Chapter 3, modern international law as a general rule outlaws resort to force
by states against other states, subject only to certain narrowly def‌ined exceptions. We
therefore begin with a review of the scope of the general prohibition on the use of force in
interstate relations, followed by an e xamination of the limited exceptions to that general
prohibition. We will see that the exceptions are conceived as necessary means when all
other options have been exhausted or are otherwise unavailable of securing respect by
states of their more fundamental international legal obligations.
Recall the basic prohibition set out in Article 2(4) of the Charter of the United Nations:1
“All Members shall 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 man-
ner inconsistent with the Purposes of the United Nations.” The corollary obligation is
found in Article 2(3) of the UN Charter: “All Members shall settle their international dis-
putes by peaceful means . . . .” Recall also the General Assembly’s elaboration of the
Article 2(4) principle in the Friendly Relations Declaration:
Declaration on Principles of International L aw Concerning Friendly Relations and
Co-operation in accordance with the Ch arter of the United Nations, GA Res 2625
(XX V), UN Doc A/RES/2625/XXV, reprinted in UN GAOR, 25th Sess, Supp No 28 at
121–24, UN Doc A/8028 Corr 1 (1970); (1970) 9 ILM 1292
The principle that States shall refrain in th eir international relations f rom the threat or use
of force against t he territorial integrity or polit ical independenc e of any State, or in any othe r
manner inconsist ent with the purpose of the United Nati ons:
1 Charter of the United Nati ons, 26 June 1945, Can TS 1945 No 7, in force 24 October 1945 [UN Charter] .
Every State has the duty to refrain in its 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. Such a threat or use
of force constitutes a violation of international law and the Charter of the United Nations
and shall never be employed as a means of sett ling international issues.
A war of agg ression constitutes a crime aga inst the peace, for which there is respon-
sibility under inter national law.
In accordance with the purposes and principles of the United Nations, States have the
duty to ref rain from propagand a for wars of aggression .
Every State has the duty to refrain from the threat or use of force to violate the existing
international boundar ies of another State or as a means of solving international disputes,
including terr itorial disputes and problems concerning frontiers of States . . . .
States have a duty to refrain from acts of reprisa l involving the use of force.
Every State has the dut y to refrain from any forcible act ion which deprives peoples
referred to in the elaboration of the princ iple of equ al rights a nd self-determination of
their right to self- determination and freedom and independence.
Every State has t he duty to refrain from organizing or encouraging the organization
of irregular forces or armed bands including mercenaries, for incursion into the territory
of another St ate.
Every State h as the duty to ref rain from organizing, instigating, assistin g or partici-
pating in acts of civil strife or terrorist acts in another State or acquiescing in organ ized
activities withi n its territory directed towards the commission of such acts, when the acts
referred to in the present paragraph involve a th reat or use of force.
The terr itory of a State shall not be the object of military occupation resu lting from
the use of force in contravention of the provisions of the Char ter. The territory of a State
shall not be the objec t of acquisition by another State resulting from t he threat or use of
force. No ter ritorial acquisition resu lting from the threat or u se of force shall be recog-
nized as legal. Nothin g in the foregoing shall be construed as affect ing:
(a) Provisions of t he Charter or any international agreement prior to the Charter re-
gime and valid under inter national law; or
(b) The powers of the Securit y Council under the Charter . . . .
Nothing in the foregoing paragraphs shall be const rued as enlarging or diminishing
in any way the scope of t he provisions of the Charter concerning c ases in which the use
of force is lawful.
While not a formal source of international law in its own right, the Friendly Relations
Declaration (also referred to as General Assembly Resolution 2625 (XXV)) is generally
accepted to b e an expression of customary international law. Consider the signif‌icance
accorded to its provisions by the International Court of Justice (ICJ) in the course of its dis-
cussion of the scope and nature of the prohibition on the use of force in the following case:
Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States
of Ame ric a), Merits, Judgment, [1986] ICJ Rep 14
[Recall that, in t his case, Nicaragua claimed that the United States had used force unlaw-
Chapter 14: The Use of Force 845
fully agai nst it, contrary to Artic le 2(4) of the UN Char ter. The following excerpts of t he
case set out the Court ’s reasoning on this issue.]
188. The Cou rt . . . f‌inds that both Par ties take the view t hat the principles as to the use
of force incorporated in the United Nations Charter correspond, in essentia ls, to those
found in customary international law. The Parties thus both take the view that the funda-
mental principle in t his area is expressed in the terms employed in A rticle 2, paragraph
4, of t he United Nations Ch arter . . . . The Court has however to be sat isf‌ied that there
exists in customary inter national law an opinio juris as to the binding character of such [a
principle]. This opinio juris may, though with al l due caution, be deduced from, inter alia,
the att itude of the Parties and the attitude of States towards certain General Assembly
resolutions, and part icularly res olution 2625 ( XXV) entitled “D eclaration on Principles
of Internationa l Law concerning Friendly Relations and Co-operat ion among States in
accordance with t he Charter of t he United Nations”. The effect of consent to the text of
such resolutions cannot be understood as merely that of a “reiteration or e lucidation” of
the treaty commitment undertaken in the Charter. On the contrary, it may be understood
as an acceptance of the validity of the rule or set of rules de clared by the resolution by
themselves. The principle of non-use of force, for example, may thus be regarded as a
principle of customary i nternational law . . . .
190. A further conf‌irmation of the validity as customary international law of the principle
of the prohibition of the use of force expressed in Article 2, paragraph 4, of the Charter of
the United Nations may be found in the fact that it is frequently referred to in statements
by State representatives as bein g not only a principle of customary inter national law but
also a fundamental or cardinal principle of such law. The International Law Commission,
in the course of its work on the codif‌ication of the law of treaties, expressed the view that
“the law of the Cha rter concerning the prohibition of the use of force in itse lf constitutes
a conspicuous e xample of a rule in international law having the c haracter of jus cogens
(paragraph (1) of the commentary of t he Commission to A rticle 50 of its draft A rticles
on the Law of Treaties, ILC Yearbook, 1966-II, p. 247). Nicaragua . . . in the present case
states t hat the pri nciple prohibiting the use of force embodied in Ar ticle 2, p aragraph
4, of the Charter of the United Nations “has come to be recognized as jus cogens”. The
United States . . . found it material to quote the v iews of scholars that t his principle is a
“universal nor m”, a “universa l international law ”, a “universa lly recognized pr inciple of
international law”, and a “principle of jus cogens” . . . .
227. The Court will apprai se the facts in the light of the principle of t he non-use of force
. . . . What is u nlawful, in accordance w ith that principle, is recou rse to either the th reat
or the use of force a gainst the territorial integrity or political indep endence of any State.
For the most part, the complaints by Nicaragua are of the actua l use of force against it by
the United States. Of the acts which the Cou rt has found imputable to the G overnment
of the United States, the following are relevant in t his respect:
the laying of mi nes in Nicaraguan internal or territor ial waters in early 1984 . . . ;
certain at tacks on Nicaraguan ports, oil inst allations and a naval base . . . .
These activities constitute infringements of the princ iple of the prohibition of the use
of force, def‌ined earlier, unless they are justif‌ied by circumstances which exclude their

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