The concept of customary international law (CIL) has been the subject of much recent reexamination, debate, and controversy.Footnote ¹ Several scholars have doubted the efficiency and effectiveness of CIL as a process to create norms in a diverse world of many different values, interests, and cultures.Footnote ² Other scholars have attempted to invigorate CIL theory with an ethical dimension to clarify norms and reduce self-serving manipulation of its elements.Footnote ³

The great advantage of CIL is that such norms are universal, applying to all nations whether they are willing to sign and ratify a treaty or not. In the traditional view, the elements of customary international law, state practice, and opinio juris are the means by which nations express their normative preferences in a decentralized system. State practice or conduct is the empirical element in customary international law formation, providing evidence of customary norms. The general acceptance of states or opinio juris determines whether a claimed norm embedded in state practice is legally binding.Footnote ⁴ Thus, it is the reaction of the states as a whole, not just that of the parties involved, that tells us how to interpret state practice.Footnote ⁵ The uniformity and generality of state practice may be an indication of opinio juris, that is, that a norm has been generally accepted as legally binding, but it is suggestive, not determinative.Footnote ⁶ The general acceptance of states also performs the critical function of legitimizing a norm by the relevant political community—the international community of states.

Professor Michael Scharf has written a thoughtful and innovative book explaining how CIL may develop with surprising rapidity in the context of fundamental change. These moments of change, which Professor Scharf refers to as Grotian Moments,Footnote ⁷ allow the accelerated formation of CIL without significant, indeed even any, state practice if there is a clear expression of opinio juris. In crafting his argument, he adopts to some degree the view of Frederic KirgisFootnote ⁸ that there is a sliding scale between state practice and opinio juris. The greater the quantity of concordant state practice, the less need for clear evidence of opinio juris. More importantly for his theory, clear evidence of opinio juris may establish a rule of CIL with little or no state practice.Footnote ⁹ For Professor Scharf, there are special contexts in times of rapid change, such as the development of rocket technology to explore outer space, that constitute Grotian moments when CIL may be formed quickly if states announce rules by which they intend to be bound.

Professor Scharf then explores six case studies testing whether a particular context may qualify as a Grotian Moment when CIL developed rapidly, or whether the proposed new rules have not yet received sufficient acceptance to qualify as CIL. The case studies include: (1) the development of the Nuremberg Principles after World War II; (2) the law governing the continental shelf after the Truman Proclamation; (3) the development of space law; (4) the law of internal armed conflict, including joint enterprise liability through international criminal courts; (5) the legality of humanitarian intervention in the shadow of the NATO intervention in Kosovo; and (6) the right to use force in self-defense against nonstate actors within other sovereign states in the aftermath of the 9/11 terrorist attack. He concludes that the first four cases were Grotian moments where CIL formed quickly, whereas in the last two (humanitarian intervention after Kosovo and the use of force against nonstate actors), subsequent state practice and a contrary view from the International Court of Justice has slowed the development of new CIL rules.

In Professor Scharf's analysis, United Nations General Assembly (UNGA) resolutions, subsequent treaties, failure to protest, and decisions of international and domestic courts play leading roles. His first case study is Nuremberg, the paradigm-shifting series of events and trials that initiated concepts such as crimes against humanity and that of individual criminal responsibility under international law. In his view, the Nuremberg Principles, which were enunciated in the Nuremberg Charter, and the judgments of the Nuremberg tribunals became CIL upon the passage of the 1946 UNGA Resolution 95(I), which unanimously affirmed the principles set forth in the Charter and judgments.Footnote ¹⁰ Prior to that expression of opinio juris, the Nuremberg Charter had been negotiated by only four states and acceded to by another nineteen states. In this view, the value of a resolution as opinio juris depends on the content and conditions of the resolution (p. 54). Here, the resolution was intended to affirm existing legal principles and was unanimous. The unique moral horror of the Nazi Holocaust, the development of Nuremberg Principles, and the establishment of an international tribunal to articulate and apply these principles had accelerated the formation of new CIL, making Nuremberg the prototypical Grotian Moment. Subsequent international, regional, and domestic tribunals have concluded that the Resolution was an authoritative declaration of CIL reinforcing this view.Footnote ¹¹

In the 1945 Truman Proclamation, the United States asserted jurisdiction over the natural resources of the continental shelf adjacent to its coastline and the determination of the boundaries with adjacent states on the basis of equitable principles. The 1981 U.S. Department of State Office of Legal Adviser legal memorandum regarding this issue defined the continental shelf as that part of the undersea land mass adjacent to the coast up to a depth of one hundred fathoms (six hundred feet).Footnote ¹² The U.S. claim was in response to new technology permitting the exploitation of offshore oil and gas resources and the growing demand for these resources and it engendered no protest from other nations. Within five years, more than thirty coastal states made claims of varying distances and types. Some, like the United States, claimed functional jurisdiction for economic or security reasons; other claimed territorial sovereignty over the sea up to two hundred miles from the coast. While the author quotes with apparent approval Sir Hersch Lauterpacht's view that given the absence of protest regarding these unilateral state declarations, the concept of the continental shelf quickly become custom by 1950; however, this conclusion is not free from doubt. The absence of protest may not have meant acquiescence. It assumes a world of perfect information, abundant legal resources, and the energy and foresight to respond to a claim that may have had little immediate impact. In 1945, World War II had only just ended, much of the world's communications infrastructure was destroyed, and there were higher priorities than responding to a rather abstract claim at the time. As the author notes, the extent of the continental shelf and the nature of these rights were not fully defined until the 1958 Convention on the Continental Shelf (p. 119). Coastal states obviously had an interest in recognizing and exploiting these resources; however, non-coastal states had an interest in minimizing the extent of the continental shelf, the territorial sea, and any exclusive economic zone. Another view is that the Truman Proclamation, rather than being rapidly recognized custom, was a unilateral claim that initiated an extended global dialogue on a series of interrelated issues, such as territorial sea claims. These claims were not hardened into law until compromises were made and accepted in the 1958 Treaty, which indeed was later modified in the 1982 Law of the Sea Convention. Such a constructivist view may not be inconsistent with the Grotian Moment concept. Fundamental change may stimulate state interaction, negotiation, and compromise.

The author's Third Grotian Moment involved the rapid improvements in rocket technology that permitted the United States and the Soviet Union to launch satellites and manned rockets into Earth's orbit and outer space. Between 1957 and 1967, there were eighty-three space flights, many of which passed over the airspace of other nations without permission. There was a vital need for a legal regime to regularize and regulate these activities, which raised novel issues such as sovereignty over celestial bodies, responsibility for space debris, and jurisdiction over spacecraft and its personnel. State practice was limited to only those two self-interested players. In the author's view, the almost total lack of protest by other states about these activities provides both state action—and inaction—and opinio juris (p. 128). In 1963, the UNGA unanimously passed the Declaration on the Principles of Outer Space. Professor Scharf poses the question: does the Declaration constitute a Grotian Moment when the principles within it have been accepted as law, or did these principles only become binding upon the later ratification of the 1967 Outer Space Treaty? In the author's view, the declaration was intended to codify CIL. The declaration was labeled “Declaration of Legal Principles” and dealt with inherently legal questions, employed the terms, “shall” and “will” indicating its mandatory rather than aspirational nature, was adopted unanimously, and its drafters indicated that it was a codification of customary law (pp. 136–37). In the author's view, the tacit acceptance of the “overflown” states quickly crystallized a new set of CIL rules. The Declaration was clear evidence of the general acceptance of these principles (opinio juris) by states, despite rather narrow state practice, and that the subsequent treaty was simply a more definite expression of rules that were already CIL. However, not all states viewed the resolution as affirming binding principles, and there are other explanations for silence in novel circumstances where rights are unknown. Even if one views the CIL process as incomplete until the 1967 Space Treaty, the brief period from 1957 to 1967 may well be considered a Grotian Moment.

In the outer space context, the Grotian Moment concept is more persuasive than in the continental shelf case. Here the general principles were contemporary accommodations to a rapidly changing reality without significant dissent that crystallized into a treaty only four years after the resolution. The continental shelf situation, by contrast, involved many conflicting claims as to both the extent of the continental shelf rights and their nature, functional or territorial. Crucially, the extent of the continental shelf requires a clear numerical standard that suggests bargaining of interests and even tradeoffs with other rights, such as the extent of the territorial sea and of an exclusive economic exploitation zone. Resolution of these many conflicting claims requires a legislative-type process where states’ conflicting interests can be balanced, rather than a contentious choice made among conflicting unilateral assertions.

Case study four involves the extension of war crimes law to internal armed conflict in the Tadić decision of the International Criminal Tribunal for the Former Yugoslavia (ICTY) and its aftermath. Prior to this 1995 decision, war crimes liability was limited to international armed conflicts. The underlying treaties, the Hague Convention of 1907 and the Geneva Conventions of 1949, remained important but were dated. By the 1990s, the location of humanitarian crises had shifted, with many of the most bloody and inhumane conflicts occurring between or among ethnic groups within a state competing for territory and power. The precipitating series of events for this Grotian Moment include: (1) the dissolution of Yugoslavia in 1991–1992 that culminated in expelling, murder, and imprisonment of 90 percent of the 1.7 million non-Serbs from the Serbian-held areas of Bosnia; (2) the genocide in Rwanda; and (3) the formation of the International Criminal Court in 1998.

Even though international humanitarian law did not expressly extend to internal armed conflicts and the terms of reference of the ICTY did not appear to cover internal conflicts, the Appeals Chamber upheld its jurisdiction over internal conflict crimes.Footnote ¹³ It reasoned that the list of applicable treaties in Article 3 of its statute was not exhaustive. It interpreted Article 3 to encompass not only Hague norms, but also violations of Common Article 3 of the Geneva Conventions and of Additional Protocol II, which it held did cover certain conduct committed in internal armed conflicts.

However, as noted by the author, two important facts undermine the ICTY's conclusion (pp. 143–45). First, the text of the ICTY Statute empowered the tribunal to prosecute persons for violations of the “laws or customs of war,” which was a term of art for violations of the Hague Convention limited to international armed conflicts. Second, the UN Secretary-General's Report that accompanied the ICTY Statute (both of which were drafted by the UN Office of Legal Affairs) limited jurisdiction to existing international humanitarian law, which is “beyond any doubt customary law.” The accompanying list included the Geneva and Hague Conventions, but not Additional Protocol II. While the decision appears unfounded, it impacted the drafting of the Statute of the International Criminal Court by prompting the inclusion of “armed conflict not of an international character.” The Statute of the Special Court for Sierra Leone contains similar language. The general opinion is that the view that IHL now also applies to at least certain internal armed conflicts is also part of customary international law.Footnote ¹⁴

In the final two case studies (the 1999 NATO intervention in Kosovo and the U.S. attack on purported members of Al Qaeda, which involved acts that were not attributed to territorial states), Professor Scharf finds no Grotian Moments. The Kosovo intervention may be conceptualized as a necessary act of humanitarian intervention to save the lives of thousands of Kosovars who were being killed or ethnically cleansed from Kosovo province.Footnote ¹⁵ While the Security Council determined that the situation in Kosovo was a threat to international peace under Chapter VII, the Council was unwilling to authorize the use of force. Several NATO states justified the bombings as a necessary humanitarian intervention compatible with Article 2(4) of the Charter (pp. 161–63).

While the NATO legal justifications at the time may be seen as evidence of opinio juris for the humanitarian intervention norm, the author concludes that that the intervention did not represent a Grotian Moment that led to the creation of a lawful exception to Article 2(4). While a Security Council resolution introduced by Russia to condemn the NATO bombing as a violation of the Charter was defeated 12–3, two permanent members voted in favor, Russia and China. As the author notes, several NATO countries, including the United States and the UK, subsequently retreated from characterizing the action as lawful humanitarian intervention that might become an open-ended right to intervene. Rather, it was now viewed as a unique situation that might be justifiable, but not necessarily legal, when the Security Council is blocked. The author reinforces this conclusion by citing the 2004 High-Level Panel Report and the 2005 World Summit Outcome Document, endorsed by both the General Assembly and the Security Council, each of which concluded that “humanitarian intervention is only lawful when authorized by the Security Council” (pp. 173–74). As the author notes, the humanitarian intervention doctrine has been reconceptualized as the Responsibility to Protect doctrine, to similar effect (p. 181). That is, states, as an aspect of sovereignty, have a responsibility to protect victims of serious human rights violations within their territory. Where a state is unwilling or unable to exercise that responsibility, it is the responsibility of the international community to act. The author is clearly correct that the 1999 Kosovo intervention did not lead to recognition of a Grotian Moment. At this point, nations continue to perceive that the dangers of abuse outweigh the benefits of recognition of a broad right to intervene. The recent Russian interventions in Crimea, which has become a de facto annexation, and in Eastern Ukraine, where Russia has been undermining the territorial sovereignty and political independence of Ukraine under the guise of humanitarian intervention, demonstrate the wisdom of the ad hoc approach to intervention.

The final case study involves the rise of international terrorism by nonstate actors sheltered in foreign states. The author asks whether the coordinated Al Qaeda attacks against the United States on 9/11, and the international community's reaction to these attacks, generate a Grotian Moment leading to new rules of CIL permitting the use of force against nonstate actors. The traditional view was that unless the nonstate actors were effectively controlled by or otherwise attributable to the territorial state, victim states could not use force in retaliation to such actions in another state. As is well-known, after 9/11, President Bush announced that the United States would exercise its right to self-defense against nonstate actors and states that harbor them. While the Bush doctrine as a political formulation may be overly broad, the Security Council appeared to approve of U.S. action when it passed Resolution 1373 condemning the 9/11 attacks, thus reaffirming the U.S. right of self-defense in the circumstances, and asserting that states are prohibited from allowing their territory to be used as a safe haven for terrorist groups. Both NATO and the Organization of American States passed resolutions supporting the U.S. view. After the United States attacked Al Qaeda camps and Taliban military targets within Afghanistan, no nation condemned the U.S. action and many affirmatively supported these actions.

Is this evidence of opinio juris sufficient to be a Grotian Moment changing CIL to allow states to use force against nonstate actors harbored in another state regardless of control or attribution? After citing the international organization supporting resolutions above and the lack of protest, Professor Scharf cites the subsequent International Court of Justice (ICJ) opinions in the 2004 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory Advisory Opinion and the 2005 Armed Activities on the Territory of the Congo case as only supporting a customary international law right to use force against nonstate actors within another state when the armed attack is attributable to the territorial state (pp. 185–86).Footnote ¹⁶ He then concludes that these conflicting currents suggest that a norm permitting states to use force against nonstate actors regardless of attribution to the territorial state is not yet CIL (pp. 208–10). On the other hand, there are well-reasoned separate opinions in both cases (cited by the author) that would permit self-defense against independent nonstate actors in such circumstances.Footnote ¹⁷

The apparent approval of this principle by the UN Security Council, its Rapporteur in 2010, regional organizations, and the absence of protest might be seen as ample indication of opinio juris. Indeed, no other potential Grotian Moment, with the exception of Nuremberg, enjoyed such broad support by states. There have been many incidents of state practice in this respect without significant objection, some of which are listed by the author (pp. 204–05). Since the book's publication, there continues to be numerous incidents of state practice with minimal protest, such as the many incursions by Kenya forces into Somalia after terrorist attacks on its soil by al Shabaab. Many of these interventions in Somalia, Pakistan, Yemen, Afghanistan, and the Congo were against nonstate actors where there is effectively no government, or where the government does not function in particular provinces. This reality demonstrates the value of the UN Special Rapporteur Philip Alston's formulation grounded in the law of neutrality: targeted killings do not violate international law where “the second State is unwilling or unable to stop armed attacks against the first state launched from its territory” (p. 197). What might be controversial is not the author's well-crafted Grotian Moment concept, but his implicit view that ICJ opinions are entitled to more weight than the views of states in the formation of CIL. As a general matter, the author gives considerable weight to non-binding UNGA resolutions, the failure to protest, and ICJ opinions, rather than the views of states in the formation of CIL. Silence does not necessarily indicate assent—particularly when the legal implications are unclear.

While perhaps the author's use of CIL material might be controversial, it does not in any way diminish his fine achievement in creating a new conceptual framework. He has crafted a new explanation of how CIL norms can emerge quickly in times of fundamental change from technological innovation to humanitarian crises. This is an innovative book that challenges our understanding of CIL development. Professor Scharf's gifts to us include a different lens into the past that illuminates “constitutional moments” in international law and an organizing method to understand the current stage of evolving norms during their formation. It is an imaginative work that gives hope in our fractious world that CIL can respond quickly to new circumstances.