Customary international law (CIL) is widely recognized as a fundamental source of international law. While its continued significance in the age of treaties was once contested, it is now generally accepted that CIL remains a vital element of the international legal order. Yet CIL is also plagued with conceptual and practical difficulties, which have led critics to challenge its coherence and legitimacy. In particular, critics of CIL have argued that it does not meaningfully affect state behavior. Traditional CIL scholarship is ill equipped to answer such criticism because its objectives are doctrinal or normative—namely, to identify, interpret, and apply CIL rules, or to argue for desirable changes in CIL. For the most part, that scholarship does not propose an explanatory theory in the social scientific sense, which would articulate how CIL works, why states comply, and why and how rules change.

When the contracting parties to the Rome Statute establishing the International Criminal Court met in Kampala in 2010 to discuss possible amendments to the statute, the main focus was, and has thereafter remained, on the crime of aggression. In addition to amending the statute to include the crime of aggression, however, the contracting parties amended Article 8 of the statute to include a broader range of war crimes in noninternational armed conflicts over which the ICC can have jurisdiction—inter alia, by including the use of chemical weapons. Although the latter amendment received much less attention from both the Kampala drafters and outside observers than the former, it is the use of chemical weapons that has come most quickly into play in world events. In particular, the use of chemical weapons by Syrian government forces in 2013 (and perhaps subsequently) has acutely raised questions concerning the extent of the ICC’s treaty-based jurisdiction, both under the unamended text of the Rome Statute or in situations where the amendment to Article 8 applies. These events have also provoked consideration concerning the Security Council’s legal powers to extend the ICC’s jurisdiction to certain crimes involving chemical weapons that would otherwise be beyond its subject matter jurisdiction. These questions are considered in this Note.

The Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on Their Destruction of 1993, or Chemical Weapons Convention (CWC), represented a major (and at the time, not uncontroversial) innovation in global treaty making. Covering a whole class of weapons of mass destruction, it mandated an immediate and comprehensive ban on use and acquisition, coupled with phased destruction and buttressed by an extensive and intrusive verification system administered by a substantial and specialized international organization. The goal was the total elimination of this particular type of weapon.