How Customary International Law Has Evolved Over Historical Periods

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The Evolution of Customary International Law

With the invention of cell phones, Internet, and ever-advancing technology, society today has slowly but surely evolved into a global village. As commercial trade and immigration become increasingly fluid between nations, international laws hold increasing weight and relevance in defining boundaries and acceptable behaviors among nations. The definition and use of customary international law, in particular, has evolved significantly since the twentieth century. From The Paquete Habana, Article 38 of the Statute of the International Court of Justice, the Lotus case, the North Sea Continental Shelf cases, and Nicaragua v. United States, the development of customary international law can be traced from initial recognition to full-fledged boundaries and definitions governing the application of customary international law and its relationship with international treaties.

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At the dawn of the twentieth century, the Supreme Court of the United States recognized customary international law as binding in an early landmark case, The Paquete Habana. During the Spanish-American War, the United States captured two Spanish fishing vessels from Cuba’s territorial waters and auctioned off both ships for monetary value. Appealing to federal courts, the owners of the vessels cited traditions as far back as orders from Henry IV of England in 1403 to show that, historically, nations had excluded fishing vessels from capture as prizes of war. In its majority opinion, the Supreme Court concurred: “For this purpose, where there is no treaty and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages of civilized nations, and, as evidence of these, to the works of jurists and commentators…for trustworthy evidence of what the law really is” (Paquete Habana, page 65). Even though no written treaty or international law existed to justify the ship owners’ appeals, the Supreme Court accepted “customs and usages” as evidence of binding international law, setting the stage for today’s internationally acknowledged definition of international law, which was formalized in 1945 when the International Court of Justice was established under the United Nations. In Article 38 of the Statute of the International Court of Justice, “international custom, as evidence of a general practice accepted as law,” is notably listed as one of four sources of international law, in addition to treaties, the generally recognized body of law, and precedents (Article 38).

Twenty-seven years after the United States recognized customary international law as binding in The Paquete Habana, the bilateral Lotus case further clarified when express permission or prohibition from customary international law was needed. This international case revolved around whether or not Turkey could prosecute a French lieutenant who had been aboard a French steamer that collided with a Turkish steamer. Because the lieutenant had remained on the French steamer, and, therefore, French territory, France argued that Turkey held no jurisdiction over the lieutenant, unless customary international law expressly stated otherwise. Conversely, Turkey argued that, unless expressly prohibited, prosecution of the lieutenant was allowed. The majority opinion explained:

The French Government contends that the Turkish Courts, in order to have jurisdiction, should be able to point to some title to jurisdiction recognized by international law in favour of Turkey. On the other hand, the Turkish Government takes the view that Article 15 allows Turkey jurisdiction whenever such jurisdiction does not come into conflict with a principle of international law (Lotus, paragraph 41).

Ultimately, the majority ruled that it did not find any customary international law that would have prohibited Turkey from exercising jurisdiction over French territory. In siding with Turkey, the court set a precedent for allowing all actions by sovereign states, barring an explicit prohibition. As the dissent stated, “Failing a rule of positive law, the relations between States in the matter under consideration are governed by an absolute freedom…a principle of public international law is set up that where there is no special rule, absolute freedom must exist” (Lotus dissent, paragraph 207).

Besides determining that only explicit prohibition is needed to restrain sovereign actions, international courts also faced the task of determining what exactly constituted customary international law. Eventually, a two-pronged test developed: customary international law must be founded upon consistent state practice and opinio juris, or belief in legal obligation. The International Court of Justice explicated this two-pronged test in the 1969 North Sea Continental Shelf case, stating, “Not only must the acts concerned amount to a settled practice, 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 a rule of law requiring it” (Continental Shelf, paragraph 77). Regarding practice, the Continental Shelf case established that a custom may not necessarily require a lengthy amount of time to become consistent state practice, as the majority wrote: “It might be that, even without the passage of any considerable period of time, a very widespread and representative participation in the convention might suffice of itself, provided it included that of States whose interests were specially affected” (Continental Shelf, paragraph 73). Moreover, in Nicaragua v. United States, the International Court of Justice ruled that evidence of state practice does not need to be completely perfect: “The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule.” Rather, as long as deviations from that practice are considered “breaches of that rule,” rather than “recognition of a new rule,” general practice can still be established. Practice alone, however, is not sufficient to determine customary international law. States must also display opinio juris, or belief in a legal obligation to follow that practice, as the Continental Shelf majority opinion declared: “Not only must the acts concerned amount to a settled practice, 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 a rule of law requiring it” (Continental Shelf, paragraph 77). If consistent state practice and opinio juris are both proven, then the existence of customary international law can be ascertained.

Along with establishing the two-pronged test for customary international law, Continental Shelf also demonstrated that a formal treaty can “embody or crystallize” “pre-existing or emergent rule of customary law” (Continental Shelf, 69). On another hand, customary international law can also arise after ratification of a treaty, “partly because of its own impact, partly on the basis of subsequent State practice” (Continental Shelf, paragraph 70). If rules of a treaty become custom, then the customary law can be shown to apply to all states, not just states that signed the treaty. Furthermore, Nicaragua established in 1986 that customary international law and treaty law are not mutually exclusive, but can co-exist at the same time. Responding to the United States’ argument to the contrary, the Court wrote, “even if the customary norm and the treaty norm were to have exactly the same content, this would not be a reason for the Court to hold that the incorporation of the customary norm into treaty-law must deprive the customary norm of its applicability as distinct from that of the treaty norm” (Nicaragua, paragraph 177). Thus, if certain countries sign conventions that concretize pre-existing customary international laws, countries, whether included in or excluded from the conventions, may still be bound to customary international law established through consistent state practice and opinio juris.

The cases and statutes from The Paquete Habana in 1900 till Nicaragua v. United States in 1986 demonstrate the rapid evolution of customary international law from its acknowledgment to boundaries and definitions governing its application, and its relationship with international treaty law. Even today, new cases are still tried and decided in the International Court of Justice, setting more precedents for international lawyers to study and analyze in years to come. Regardless of whatever trends customary international law may follow, the abovementioned cases will still remain as landmark precedents instrumental in the development of international law. As unprecedented inventions bring nations and communities increasingly closer, global rules will become more necessary than ever, to govern a global village.

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