The Use of Force in International Relations

AuthorJohn H. Currie
C H A P T E R 1 1
In this chapter we consider the international legal rules governing
whether, and under what circumstances, states may resort to armed
force in t heir international relations. This area of international law is
of relatively recent vintage, and stands, along with the emergence of
international human rights law, as one of the most profound twentieth-
century innovations in substantive international law. Its emergence is
also inextricably entwined with modern international law’s most im-
portant institutional development, the establishment of the United Na-
tions (UN) and in particular the UN Security Council. However, given
that it strikes at the very heart of what historically was considered a
sacred prerogative of state sovereignty — the right to make war — inter-
national law governing the use of force is a profoundly contentious
area. Moreover, as a st ate’s decision to resort to armed force is usually
driven by a domestic assessment that some fundamental national inter-
est requires it, important questions arise as to the effectiveness of rules
of inter national law that would seek to constrain such decisions.1 In
short, familiarity with the law governing resort to force is helpful in
understanding the modern international legal system more generally,
as well as its potential shortcomings.
1 See, generally, C. Gray, Intern ational Law and the Use of Force, 2d ed. (Oxford:
Oxford Universit y Press, 2004) c. 1.
The Use of Force in Internat ional Relations 4 49
Before examining the history, extent, and limits of modern inter-
national law’s prohibition of the threat or use of force, it is important
to underline a fundamental legal distinction. This chapter concern s
itself with rules governing whether and when states may resort to
armed force. In that sense, the focus of such rules is the threshold issue
of whether the use of force is in pr inciple legally permissible in any
given situation. This is ref‌lected in the expression jus ad bellum (lit-
erally, “right to war”), which is commonly used to descr ibe this area of
international law. These rules are to be distinguished, however, from
a closely related yet fundamentally distinct area of international law:
the law of armed conf‌lict, sometimes also referred to as international
humanitarian law or the jus in bello (literally, “law in war”). In con-
trast to the jus ad bellum, the jus in bello governs the manne r in which
armed force may be used once it has been resorted to.2 While there is
an import ant academic debate on the relationship between both bod-
ies of law and how they may or should interact, the preponderance of
state, scholarly, and judicial practice is to treat them as rigidly distinct.3
In this way, violation of the jus ad bellum is generally not considered
relevant to the ongoing existence of, or assessment of respect for, obli-
gations under the jus in bello, and vice versa.4 While consideration of
the jus in bello a vast and important area is beyond the scope of
this chapter and book, the reader is referred to the copious literature
addressing the topic.5
2 Caution is requi red in thinking of the ju s ad bellum as solely concerned wit h
“threshold” determ inations of the legality of r esort to force. Such a character iza-
tion should not be mist aken to mean that once a state f‌ir st uses force, the jus ad
bellum is spent and be comes irrelevant to asse ssing the legality of sub sequent
uses of force by th at state in the course of the same c onf‌lict. Rather, state prac-
tice and schola rly opinion appear to accept that eac h instance of a state’s use of
force in an ongoing conf‌lict i s subject to legal assessme nt according to the jus
ad bellum (in addition, of cours e, to the jus in bello): see C. Greenwood, “The
Relationship of Iu s ad Bellum and Ius in Bello” (1983) 9 Rev. Int’l Stud. 221.
3 See C. Greenwood, “H istorical Development and Lega l Basis” in D. Fleck, ed.,
The Handbook of Huma nitarian Law in Armed Conf‌licts (New York: Oxford Uni-
versity Pre ss, 1995) 1 at 7–8.
4 M. Sassòli & A. A. Bouvier, How Does Law Protect in War?, 2d ed. (Geneva: ICRC,
2006) at 102–8.
5 Recent contribut ions to this literature i nclude Y. Din stein, The Conduct of
Hostilities Under the L aw of International Arme d Conf‌lict (Cambr idge: Cambridge
University Pre ss, 2004); D. Fleck, ed., The Handbook of Humanitaria n Law in
Armed Conf‌licts, 2d ed. (Oxford: O xford University Press, 20 08); L.C. Green,
The Contemporary Law of Ar med Conf‌lict, 2d ed. (Manchester: M anchester
University Pre ss, 2000); J-M. Henckaerts & L. D oswald-Beck, Customary Int er-
national Huma nitarian Law (Cambridge: Cambr idge University Press, 200 5);
We turn now to our examination of the jus ad bellum — that is, the
rules of international law governing the threshold issue of whether and
when a state may resort to armed force in the f‌irst place.
While it is commonplace to consider the international legal prohibition
of the threat or use of force as a post-Second World War development
coinciding with adoption of the UN Charter and the establishment of
the UN, this is a considerable and not-altogether accurate oversimplif‌i-
cation. This is relevant because an understanding of the meaning of the
UN Char ter’s provisions on the use of force, and of their relationship
with customary international law, turns in part on an appreciation of
the pre-1945 international legal context.
In fact, if one takes the long view, attempts to constrain resort to
war by the rule of law have an ancient pedigree, and the general liberty
to use armed force that characterized eighteenth and nineteenth cen-
tury international law emerges as a relatively f‌leeting and aberrant phe-
nomenon. In other words, it is not accurate to think of legal limitations
on the use of force as an exclusively mid-twentieth century innovation.
Nor does history bear out the sometimes-advanced assertion that war,
by its very nature, is beyond the scope of legal control.6
Certainly there is clear evidence that several ancient civilizations
placed legal constraints on resort to armed force.7 W hi le as su m in g m an y
forms, for the most part these constraints required that war only be in-
voked in response to some prior violation of a right or in some other
just cause. Perhaps the best-preserved evidence of such requirements
F. Kalshoven & L. Zegveld, Co nstraints on the Waging of War, 3d ed. (Geneva:
ICRC, 2001); H. McCoubrey, International Humanitar ian Law, 2d ed. (Aldershot,
UK: Ashgate, 1998); L. Moir, The Law of Inter nal Armed Conf‌lict (Cambridge:
Cambridge Univer sity Press, 2002); R. Provost, Inter national Human Rights
and Humanitar ian Law (Cambridge: Cambridge Univer sity Press, 2002); A.P.V.
Rogers, Law on the Bat tlef‌ield, 2d ed. (Manchester: Ma nchester University Pres s,
2004); Sassòli & Bouv ier, ibid.; UK Min istry of Defence, The Manual of the L aw
of Armed Conf‌lict (Oxford: Oxford Un iversity Press, 2004).
6 See Y. Dinstein, Wa r, Aggression and Self-Defence, 4th ed. (Cambri dge: Cam-
bridge Universit y Press, 2005) at 73–75.
7 See, for example, I. Brow nlie, International L aw and the Use of Force by States
(Oxford: Clarendon Pre ss, 1963) at 3–5; Din stein, above note 6 at 63–64; Green-
wood, above note 3 at 12–15.

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