Customary international law is an aspect of international law involving the principle of custom. Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of international law.

Many governments accept in principle the existence of customary international law, although there are differing opinions as to what rules are contained in it.

In 1950, the International Law Commission listed the following sources as forms of evidence to customary international law: treaties, decisions of national and international courts, national legislation, opinions of national legal advisors, diplomatic correspondence, and practice of international organizations.


The judgments of the permanent court of international justice (PCIJ) and the ICJ have been consistent in stating that a customary rule requires the presence of both of these elements. Thus, in the 1929 S.S. Lotus, the PCIJ stated that international law is based on the will of states expressed in conventions or in “usages generally accepted as expressing principles of law”.

Similarly, in the 1969 North Sea continental Shelf, the ICJ stated that the actions by states “not only must amount to a settled pratice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of the rule of law requiring it”.Where those two elements are manifest, a rule of customary international law, will be deemed to bind all states, with the exception of persistent objectors, without it being necessary to show that the particular state allegedly bound by the rules has participated in its formation or has other wise accepted it. Article 38(1)(b), as quoted above, provides the most authoritative definition of international custom (Cassese 2005,156)even if not an undisputed one (Kunz,1953,664), describing  it  as  evidence of a general practice accepted  as law. This formation reflects the widely accepted two–element theory that custom is composed of two elements: State practice and opinion juris, both of which will be further explored in the next section of this study.

1. The objective element (general state practice)

Traditionally, jurists and scholars have put more emphasis on state conduct than on the subjective element. That is because a state conduct was traditionally easier to ascertain than the belief of a state. With the introduction of the United Nation (UN) and other bodies where multilateral diplomacy is conducted in the open, however, the situation has in fact reversed. State practice can be reflected in the acts of the judiciary, legislation or executive branch of government; declarations of government policy; the advice of governments  legal advisers, press statements military manuals, votes and explanation of votes in international organizations, the comments of governments on draft texts produced by the ILC, national legislation, domestic court decisions; and pleadings  before international tribunals.While one might be tempted to conclude that acts count more than words because “talk is cheap”, virtually all of the authorities treat the two as equal.In fact, ICJ judge Richard Baxter once noted the firm statement by the state of what it considers to be the rule is far better evidence of its position than what can be pieced together from the actions of that country at different times and in a variety of contexts.The case law of international tribunals is replete with examples of verbal acts being treated as examples of practice. In particular, diplomatic protest (or its absence) is universally viewed as important in determining whether a customary rule has been created or superseded. Thus, in assessing the relevant behavior of states, “we look to words as well as deeds , and to silences as well as inactions” . Verbal acts can count as either the objective or subjective element and  the International Law Association (ILA) has observed that it is possible for the same conduct to manifest both.As discussed below, state vote on UN General Assembly Resolutions can thus be both a form of state practice and a manifestation of the state subjective attitude about the existence of the rule in question. State practice also includes inaction or silence, especially (but not necessarily) where a protest would be expected. Thus, in the case of the S.S.Lotus, the PCIJ relied on the absence of protest against legislation based on the objective territoriality “doctrine of jurisdiction in finding that such an exercise of jurisdiction was permissible under customary international law”.Similarly, in the Nottebohm case (second phase), the International court of justice (ICJ) based its decision on fact that some states “refrain from exercising protection in favor of a naturalized person when the latter has in fact, by his prolonged absence , severed his links with what is no longer for him anything but his nominal country… 15Professors Myers Mc Dongle described the customary international law formation process as one of continuous claim and response. To illustrate this process consider the question of whether international law permits a state to use force to arrest a terrorist leader in another state without the latter consent. A question that recently arose when the United States Kidnapped an Al-Qaeda leader from Libya in October 2013. The claim may be express, such as demanding that its special forces be allowed to enter the territorial state to arrest the terrorist, or implicit, such as sending it special forces into the territorial state without it permission to apprehend the terrorist. The response to the claim may in turn be favorable, such as consenting to the operation or refraining from protesting the extraterritorial apprehension. In such case, the claim and response will begin the process of generating a new rule of customary international law. Some states may initiate the practice and others may passively acquiesce in it.

2. The subjective element (opinion juris)

The purpose of the subjective element, known as opinion juris, is to differentiate state actions that give rise to legal norms from actions that do not.

The subjective element has been described as “the philosopher’s stone which transmutes the inert mass of accumulated usage into the gold of binding legal rules”.

This element is necessary because state practice is often capable of being interpreted in various ways. Examining subjective intent is particularly important where actions (or omissions) are in and of themselves ambiguous. For example, Professor D’ Amato has said that every breach of a customary law contains the seed for a new legality.

But that is only the case if the breaching state justifies its action on the basis of a new rule of customary law. As the international court  of justice (ICJ) observed in the Nicaragua case, if instead the state resorts to factual or legal exceptions to justify the breach, this has the effect of confirming the general rule rather than undermining it or creating an exception to it.

Moreover the subjective element can reveal actions that constitute mere comity and distinguish those taken out of mere convenience from those that count as precedents. Take for example, the S.S .lotus. The France argued that the dearth of prosecutions for collisions on the high seas other than by the flap state on board which the wrongful act took place was evidence that international law did not recognize “effects jurisdiction” in such cases. The PCIJ disagreed on the ground that there was no evidence of a “conscious (ness) of having a duty to abstain’’ from prosecuting officers of ships of foreign registry for injuries to ships of domestic registry. The reasons for lack of interest or lack of domestic statutory authority as on belief that such prosecutions violated customary international law. There are two vigorously contested schools of thought concerning the nature and role played by the subjective element. The “voluntarist” thesis maintains that, since states are sovereign, they can not be bound by legal obligations (whether through treaty or customary law) without their consent. Consistent with this, voluntaristis view the subjective element of customary international law as a manifestation of consent. The competing “belief” thesis maintains that custom’s binding force is based in the states belief in the legal necessity or permissibility of the practice in question. The controversy between supporters of the two approaches goes back more than 200 years, and has continued to this day. The voluntarist thesis has been criticized for adopting the legal fiction that silence is considered a form of acquiescence or tacit consent a fiction that is particularly hard to buy in cases where the particular state was not directly affected by the conduct in question when the customary rule was being formed.The belief thesis, in turn, has been criticized for failing to explain how opinion juris can exist in the initial phase of state practice with respect to a new rule. If it is the first state to assert a new rule, how can the state seriously entertain the opinion that it is acting in accordance with the law?

Some commentators have tried to answer this criticism by suggesting that the state could be acting in error, but “the truth is that the state which introduces a new practice has in these cases no belief that its conduct is permitted or required by existing law, and neither have the first states to respond positively or negatively to its conduct”.

However some scholars believe that the dichotomy between the two approaches is a false one that consent plays a role in some circumstances, and belief does in others. Professor  Mendel son, for example, maintains that the voluntarist thesis is the better approach, which emphasizes belief , better explains why mature customary rules are observed. Consistent with words in Article 38 of the state of the International court justice (ICJ)  “a general practice accepted as law”, Mendel son suggests that in the early formation stage “acceptance ’’ means consent to an emerging rule , and in the later stage “acceptance” means acknowledgment that the rule has gained the force of law. Moreover, he stresses that such an acknowledgment need not come from states alone, but could emanate from a tribunal or from a resolution of the U.N general Assembly.

While, some scholars have argued that “customary law without custom (practice) is a contradiction in terms” the ILA has pointed out that sine statements are a form of state practice, how a state votes and how it explains its vote in the general Assembly is a form of practice that can generate customary law then if emergent practice exists outside the resolution, the adoption of the resolution can serves as a collective expression of opinion juris that can crystallize the rule.

In case of Nicaragua, the International court of Justice appears to have treated general Assembly Resolution 2625, the declaration of principles of international law concerning friendly relations and co-operation among states, as a resolution that generated customary international law. The court stated that :the effect of consent to text of such resolutions can not be understood as merely that of a “reiteration or elucidation” 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 declared by the resolutions by themselves.


Customary international law as a second formal source of international law require for its creation both a regularity of practice or state practice and an expression of act(s) of will, even if masked as opinion juries. Customary international law without the custom could still be general international law while without the act of will, it is not positive law and, in the last instance, not law (or even normative) at all. But our information on how customary international law comes about is not authoritative, merely guesswork, understanding a new of customary international law in case of Nicaragua. Several problems arise from the decision. Most generally, the Nicaragua understanding of customary international law can be criticized for its obscurity. The court’s analysis of custom illustrates the accuracy of Sir Robert Jennings observation that “we can not reasonably expect to get very far if we try to rationalize the law of today, solely in the language of Article 38 of the statute of the international court of justice”.

  1.  1945,ICJ.Stat.1055,T.S.N0 993. All member states of the united Nations are automatically parties to the statute of the International court of justice.
  2. Maurice H.MENDELSON, the formation of customary International Law 195(1998).
  3. S.S.Lotus(Fr. V. Turk),Merits, 1927 P.C.I.J.(Ser. A) N0.10,at 18(Sept.7).
  4. North Sea Continental Shelf, 1969 I.C.J. 77.
  5. Mendel son, Supra not 27, at 218.
  6. Cassese, Antonio .International  Law .New York :Oxford University Press,2005.
  7. Kunz, Josef L.“The nature of Customary International Law”,October 1953:662-669.
  8. Id.at197.
  9. I A N Brown Lie , Principles of Public International Law 5(4th ed 1990)
  10. INT’L Law ASS’N, Supra note 16,at 13-14.
  11. Richard Baxter , Multilateral Law Treaties as Evidence of customary International Law,41 BRIT .Year Book of INT’L L.275,300(1965-1966).
  12. See S.S.Lotus,1927 P. C. I. J. at 23;26-30  Nottebohm case (Lich.V.Guat),1955 I.J.4,21-23( Apr.6); Fisheries Jurisdiction (U.K.V.Ice), Merits , 1974 IC.J 3, at 24-26;55-58(July25); Military and Paramilitary Activities in and Against Nicaragua (Nicar.V.U.S).
  13. Kop low , Supra note 17, at 160.
  14. INT’L Law ASS’N, Supra  not 16, at 7.
  15. Mendel son , Supra not 27, at 201.
  16. S.S.Lotus,1927.P.C.I.J. at 23
  17. Nottebohm case ,1955 I. C .J. at 22
  18. See Generally M. S. Mc Douglas &N.A Schalei , The hydrogen bomb Tests in perspective: Law full Measures for security , 64 yaleL.J.648(1955)
  19. Ernesto .Londoner, Capture of bombing suspect in Libya represents “rendition” by U.S Military , Washington Post (oct.6,2013), available at http:// articles .Washington – Kerry-terror suspects-Libyan-government (last visited feb.22,2014).
  20. INT’L Law ASS’N, supra not 16 at 10.
  21. H. Thirlway , International Customary Law and codification 47(1972).
  22. D’Amato, supra note 45, at 97-98.
  23. Nicaragua case,1986I.C.J at 98,186.
  24. S.S.Lotus ,1927 P. C .I. J. at 28.
  25. Id.
  26. Mendel son ,supra note 27, at 246 n.244(citing diametrically opposed articles).
  27. INT L. Law ASS’N, supra note 16, at 39.
  28. Mendel son, supra note 27 , at 280.
  29. Mendel son, supra note 27 at 248.
  30. Id. at 283.
  31. Id.
  32. Id.  At 396 ( Mendel son points out that in “certain innovative decision such as the 1951 Fisheries jurisdiction case, the reservations to the genocide convention cases , and the Nottebohm case, it would not be far from the truth to say that the ICJ made new Law”).
  33. Mendel   son , supra note 27, at 283
  34. Id. at 41, Id. at 99;188.