Customary international law arises from the practices of nations followed out of a sense of legal obligation. Although long an important source of international law, there continues to be debate and uncertainty about customary international law’s status in the US legal system. The US Constitution makes clear that treaties are part of the “supreme law of the land,” and the US Supreme Court has explained that this means that a treaty can displace contrary state law, and override an earlier-in-time federal statute, if the treaty is “self-executing.” Treaties, in other words, are part of the federal, national law of the United States. By contrast, the only reference to customary international law in the Constitution is a grant of power to Congress to define and punish offenses against the “law of nations,” a phrase that would have encompassed what we today call customary international law.
The limited reference to customary international law in the Constitution might suggest that this body of law is part of the national law of the United States if and only if Congress incorporates it by statute. That is, the constitutional text can be read to suggest that, unlike treaties, customary international law is never self-executing federal law. Such a conclusion might draw support from considerations of democratic process. Changes to domestic law, it can be argued, should be considered and approved by the elected branches of the government. This is the case for treaties: to become domestic law, they require the approval of the President and two-thirds of the Senate, and even then they are judicially enforceable only if self-executing. Customary international law, by contrast, is not directly approved by the elected branches of the government.
Throughout the nineteenth and early twentieth centuries, US courts applied customary international law without requiring that it first be codified by Congress and even referred to it as “part of our law.” Importantly, however, these courts did not view customary international law as superseding otherwise applicable domestic law. Instead, they treated customary international law as having the status of “general common law” – that is, as law that was neither federal law nor state law, and which could be applied only in the absence of contrary domestic law. In any event, the practice of federal courts applying general common law was eliminated by the US Supreme Court in its landmark 1938 decision, Erie Railroad v. Tompkins, in which the Court stated that “[t]here is no federal general common law” and that “[e]xcept in matters governed by the Federal Constitution or by Acts of Congress, the law to be applied in any case is the law of the State.”
Since Erie, the federal courts have developed discrete bodies of “federal common law” – that is, judicially developed rules that have the status of federal, national law (and which, therefore, displace contrary state law). The Supreme Court held in its 1964 Sabbatino decision, for example, that the “act of state doctrine,” whereby courts are to treat as valid the acts of foreign sovereigns taken within their territories, is a rule of federal common law binding on state courts. Some commentators have argued that all of customary international law should, like the act of state doctrine, be viewed as having the status of federal common law. If accepted, this argument would mean that courts would have broad authority to invalidate state laws deemed inconsistent with emerging norms of customary international law – perhaps, for example, concerning the death penalty. It might even mean that courts could enforce customary international law against the executive branch, and potentially even use it (as they can self-executing treaties) to override earlier-in-time federal statutes.
In practice, US courts have not applied customary international law in this way. Courts have not applied customary international law to displace state law, and they have affirmatively rejected efforts to apply customary international law to override actions of the executive branch and Congress. In the 1980s and 1990s, a few lower courts did say, in human rights cases brought under the Alien Tort Statute, that claims for violations of customary international law presented federal law questions sufficient to give the federal courts jurisdiction over these questions. That jurisdictional issue became moot after the US Supreme Court’s 2004 decision in Sosa v. Alvarez-Machain, which reasoned that the Alien Tort Statute should be understood as a “limited, implicit sanction” from Congress for the courts to develop a modest number of federal common law causes of action in support of the Statute. Under this reasoning, the federal law in Alien Tort Statute cases is coming from a congressional delegation of authority, not from any inherent domestic status of customary international law. (Such federal law has an even narrower field of application after the US Supreme Court’s 2013 Kiobel decision, which substantially restricted the extraterritorial reach of Alien Tort Statute litigation.)
Even if customary international law does not have the status of self-executing federal law, it does not mean that its role in the US legal system is unimportant. Congress can incorporate customary international law into US law, and it has done so in a number of ways, such as by codifying the standards for foreign sovereign immunity, providing a civil cause of action for acts of torture committed under color of foreign law, and allowing for criminal prosecution of acts of piracy “as defined by the law of nations.” Courts also have long applied a canon of construction – known as the “Charming Betsy canon” – whereby they will construe federal statutes, where reasonably possible, to avoid violations of customary international law. In addition, it is possible that in developing some rules of federal common law relating to foreign affairs, courts will take account of a mixture of international law and domestic law considerations, even if they do not apply customary international law directly. This may be happening, for example, with respect to the law governing the immunity of foreign government officials, something that the US Supreme Court in its 2010 Samantar decision indicated should be developed as a matter of federal common law.
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