International law has struggled to regulate terrorism for over a century, beginning with efforts to cooperate in the extradition and prosecution of suspects, including through unsuccessful League of Nations efforts to define and criminalize terrorism as such. Until 2001 most international attention focused on transnational criminal cooperation against terrorism, through the development of method-specific “prosecute or extradite” treaties (concerning, for instance, violence against aircraft or ships, hostage taking, or attacks on diplomats) but without defining terrorism as a general concept or crime. It may, however, be possible to qualify some terrorist acts as war crimes or crimes against humanity. Since the 1970s, there were ambivalent efforts through the UN General Assembly to develop normative frameworks to confront terrorism per se, which often came unstuck on the controversial issues of “state terrorism” and liberation movement violence. Greater consensus was achieved by 1994 with the General Assembly’s adoption of a declaration against terrorism. There appears to exist an international consensus that terrorism per se is wrongful, even if disagreement remains about identifying precisely what constitutes terrorism. The effort to deal with terrorism as such suggests that the international community views terrorism as more than its underlying physical parts, which are already crimes in most national legal systems and under certain transnational treaties. The special wrongfulness of terrorism is perhaps signified by its intimidation of civilian populations, its coercion of governments or international organizations, and its political, religious, or ideological aspect. Terrorist violence has also sometimes raised certain problems under the law of armed conflict and the law on the use of force, as well as occasionally attracted sanctions imposed by the UN Security Council. Terrorism was generally dealt with, however, through the application of general legal norms rather than through the emergence of terrorism-specific rules. After the terrorist attacks of 11 September 2001, sharper international focus was brought to bear on the legal challenges presented by terrorism and counter-terrorism in numerous specialized branches of international law (particularly in the law of state responsibility, the law on the use of force, and international humanitarian law), as well as in the institutional practices of the UN Security Council and the impacts of counter-terrorism measures on international human rights law. By 2011 the UN Special Tribunal for Lebanon even declared the existence of an international customary law crime of transnational terrorism, although that decision has proven highly controversial as not supported by state practice. Efforts to negotiate a comprehensive international convention against terrorism have continued since 2000, with disagreement remaining over the scope of exceptions. There is also now increasing debate about whether a field of international anti-terrorism law is emerging.
Use of Force in Cyberspace