Anthony Bellia of the University of Notre Dame and Bradford Clark of George Washington University have written a fascinating, innovative study of the relationship between the US Constitution and customary international law. They have identified an important problem of pressing concern and, working within a broadly originalist framework, have offered an elegant and persuasive solution to it.

The problem that Bellia and Clark explore is how to fit the “Law of Nations” (as it was called in the eighteenth century) or “customary international law” (as it is known today) together with the constitutional law of the United States.

What is customary international law? According to the Statute of the International Court of Justice (ICJ), it is the “general practice [of States] accepted as law.” More fully, according to the ICJ, there are two main criteria of customary law: “settled [state] practice together with opinio juris.” “Opinio juris” refers to the belief that the practice in question is rendered obligatory by the existence of a rule that requires it.

“Customary” international law is distinct from “conventional” or treaty law. Their relationship is similar to that between the “common” law and “statutory” law. (However, even if incorporated into treaty law, customary rules may remain binding as custom.) Significant parts of international law remain customary rather than treaty law.

Customary law reaches matters of vital importance in international affairs, e.g., the law of armed conflict, the immunity of certain officials from the jurisdiction of foreign courts and human rights law, including (it has been asserted) the right of minors to be free from parental involvement in abortion decisions, the right to a healthy environment, and the right of juvenile offenders to avoid life imprisonment without parole.

The Constitution refers explicitly to treaty law:  it lays down the procedures for the ratification of a treaty, and it postulates that treaty law is to be part of the “supreme” law of the US, prevailing over any contrary state law. But the Constitution is more reticent both about how to identify customary law and whether it forms part of our “supreme” law. Despite that, the position of customary law in our legal system has been discussed, if infrequently, in several Supreme Court opinions over the course of two centuries, including The Schooner Exchange v. McFadden, The Prize Cases, The Paquete Habana, and Sabbatino v. Banco Nacional de Cuba.

There are three main views about the relationship between customary international law and federal law. The “modern” position holds that customary international law is automatically incorporated into domestic law, and furnishes US courts with rules of decision. The “revisionist” position affirms that customary international law has no legal force in the US or its courts unless the political branches adopt it by treaty or statute. The intermediate position treats customary international law as general (but non-federal) law that does not prevail over contrary State law.

These differences matter. If customary international law were automatically incorporated into domestic law, then our courts might hold, e.g., that parents could play no lawful role in their minor daughters’ abortion decisions. Courts could also affirm that the US military was bound by a 1977 “protocol” to the Geneva Conventions, even though the US has long refused to ratify that treaty. Advocates could argue, and courts might agree, that the protocol had become customary law binding even non-parties to it, including this country. International legal rules framed by other countries (often, hostile ones) that had been rejected under the carefully wrought procedures that our Constitution prescribes could thus become domestic legal mandates.

Moreover, such laws could be held binding even if they did not reflect the actual practice of other States, but merely their statements at diplomatic gatherings. So, for instance, the Trial Chamber of the International Criminal Court for the former Yugoslavia opined in 2000 that a “customary” rule may have emerged “even where State practice is scant or inconsistent,” and that opinion juris “may turn out to be the decisive element.” In other words, a rule might be considered “customary” even in the absence of custom.

Which of the three views noted above is correct? The chief merit of this book is to show that none of them is, and that a better solution is possible. In brief, the problem with all three views is that they assume that the question of how customary law enters our domestic law has a single, unitary answer. In fact, the authors argue, there are three answers, not one.

The authors begin by pointing out that when the Constitution was framed, the Law of Nations had three main components: the law of state-to-state relations; the law merchant; and the law maritime. The Constitution enabled the federal government to interact with each of these three types of law in different ways.

The most important part of the Law of Nations consisted in the law of state-to-state relations. This body of law encompassed the norms that regulated interactions between states, including the rights and duties of neutral States in times of war, the conduct of diplomacy, the protection of a State’s nationals on foreign soil or the high seas, and the authority of a State over its own subjects. It was founded upon the respect of recognized sovereigns for one another. Compliance with this body of law was essential if peace was to be maintained and war avoided.

The law merchant comprised general commercial law, administered by the courts of various nations, with a view to facilitating commercial transactions among traders from different States. The law maritime included elements of both other kinds of the Law of Nations, such as the law of naval prize (which concerned captures of foreign vessels on the high seas) and business transactions on the high seas or near coastal ports.

The key to understanding the role of the Law of Nations in domestic adjudication, the authors argue, lies in seeing how US courts have historically applied each of these three types of law. Consider the law of state-to-state relations. The Constitution confides control over war and peace to the political branches — Congress and the President. The powers to declare and engage in war, to undertake reprisals, to authorize captures on the high seas, to make peace, and so on, were assigned specifically to the political branches. The courts’ role in matters of war and peace was subsidiary. Accordingly, the Constitution placed within the courts’ jurisdiction only selected kinds of cases that were likely to involve state-to-state relations — e.g., cases affecting ambassadors. Conscious of their constitutional limitations, the courts have not assumed the general authority to decide all cases in which the law of state-to-state relations might apply. Instead, their practice in this area has observed two “default” rules.

First, the courts have given effect to the political branches’ decision to recognize a foreign state by upholding the rights of such recognized States under the law of state-to-state relations, or otherwise deferring to the political branches’ constitutional control over war and diplomacy. The early (1812) case of The Schooner Exchange is illustrative. There the Supreme Court rejected the claim of the original American owner of a French warship anchored in Philadelphia to the recovery of the vessel. At issue was whether an American citizen could assert, in an American court, a title to a foreign warship found within American territorial waters. Obviously, ordering the seizure of a foreign warship could amount to a judicial declaration of war. Because the French ship was in US (rather than international) waters, the rights of France under the law of state-to-state relations did not directly apply. However, the Executive branch “suggested” to the courts that the French naval vessel should be held “immune” from seizure. Deferring to the political branches’ constitutional authority over war, the Court refused to order the vessel seized.

Second, in subsuming the law of state-to-state relations, the Constitution disempowered the courts from questioning the acts of a foreign sovereign on its own territory. The 1964 Sabbatino case is on point. There, the Supreme Court refused to adjudicate a claim to compensation by the owners of property located in Cuba that the Castro government had confiscated. In effect, the Court ruled that controversies between a foreign State and those owning property within that State’s territory were left to that State. The law regulating state-to-state relations generally required as much. If our political branches nonetheless chose to demand redress from Cuba for the confiscation, that question was to be decided by them, not by the judiciary.

Sabbatino falls within the long tradition of American constitutional jurisprudence concerning the Law of Nations. But it also discountenances any tendency to hold foreign sovereigns liable for human rights violations of customary law committed on their soil. To put it simply, the Constitution furnishes no basis — in the absence of a treaty or Act of Congress — for an interventionist judiciary to vindicate the human rights of foreign nationals in their home countries.

This brief review can offer only a glimpse at the richness, depth and originality of this fine book. It takes an important constitutional debate, which has been inconclusive until now, to an altogether higher level.