In the merits phase of decision in the case brought by Nicaragua against the United States, the World Court briefly mentions references by states or publicists to the concept of jus cogens. These expressions are used to buttress the Court’s conclusion that the principle prohibiting the use of force found in Article 2(4) of the United Nations Charter is also a rule of customary international law.

Central to the World Court’s mission is the determination of international custom “as evidence of a general practice accepted as law.” Students of the Court’s jurisprudence have long been aware that the Court has been better at applying customary law than defining it. Yet until Nicaragua v. United States, little harm was done. For in the sharply contested cases prior to Nicaragua, the Court managed to elicit commonalities in argumentative structure that gravitated its rulings toward the customary norms implicit in state practice. The Court’s lack of theoretical explicitness simply meant that a career opportunity arose for some observers like me to attempt to supply the missing theory of custom.

Even if conceived of only as a legal text, the array of judicial opinions contained in Nicaragua v. United States constitutes an extraordinary document. It represents a fascinating attempt through judicial inquiry to assess convincingly the relevance of law to an ongoing armed conflict. As such, despite procedural objection to entry upon this terrain by the three dissenting judges, it leads the Court to pronounce specifically upon the core issue of when force can permissibly be used in international relations, as well as the contours of a claimed right of collective self-defense in the setting of interventions and civil strife.

As a recurring feature of the Cold War that has dominated international relations for the past four decades, foreign intervention in civil armed conflicts has focused and inflamed scholarly debate over the content of the relevant legal restraints. Conflict has raged particularly around the following issues: First, what forms and degree of assistance to rebels constitute an armed attack within the meaning of Article 51 of the Charter authorizing individual and collective self-defense? Second, in cases where assistance does not reach the armed attack threshold, are there any circumstances in which the target state and/or its allies may nevertheless use forceful measures to terminate it?

The decision of the International Court of Justice in the case between Nicaragua and the United States brims with important procedural and substantive implications for the future of law and adjudication in disputes between states.