Kai Ambos and Anina Timmermann
The following chapter attempts to analyse if terrorism has become a crime under customary international law. We start with a brief look at the minimum requirements of customary law in general and its particularities regarding criminal law. It will be shown that there are several general concerns regarding the creation of criminal provision by custom, especially because of the inevitable vagueness of unwritten law and the ensuing conflict with the principle of legality (nullum crimen sine lege). We will then explain the different categories of international(ized) crimes, distinguishing between the core and the treaty-based crimes. The second section will focus on the crime of terrorism. We will inquire into the relationship between terrorism and international criminal law and how it is affected by custom. Other international law aspects, intimately related to the current terrorism debate, such as state responsibility, the duty to prevent terrorism, state territory being used by terrorists, and the lawfulness of the use of force against terrorism, will not be treated. The gist of the issue is whether terrorism fulfils the decisive criteria of a true international crime, thereby making it possible to prosecute perpetrators globally, notwithstanding their protection by states. We will, on the basis of several international treaties, suggest a possible customary law definition of terrorism. We will conclude though that at the current state of international law, terrorism can only be qualified as a particularly serious transnational, treaty-based crime that is, at best, on the brink of becoming a true international crime but has not achieved this status yet.