It is remarkable that two decades of extensive global counterterrorism law and cooperation have proceeded from a normative black hole: the absence of a common definition of terrorism. Security Council Resolution 1373 and successive resolutions have deliberately omitted any definition, despite requiring states to take far-reaching legislative and executive action.

On the one hand, the Council’s approach was tactically brilliant. Since new counterterrorism measures were perceived to be urgent after 9/11, there was no time to get bogged down in the intractable question of definition, which had eluded international agreement for a century. The limited club of 15 Council members would have been unlikely to agree. Even if they had, their definition would have been unlikely to reflect or attract an international consensus, triggering serious compliance problems in national implementation. 

Despite “making” new law, as a political body the Council seemed satisfied with its political instinct, expressed by the then United Kingdom’s Ambassador, Jeremy Greenstock, that “What looks, smells and kills like terrorism is terrorism.” Legal definition was instead devolved to states in national implementation. While the Council intrusively demanded extensive domestic legislation, individual states were generally content to retain their sovereign discretion to identify and legislate for themselves the meaning of terrorism. 

Much valuable and principled counterterrorism law and cooperation has occurred based on the Council’s suite of resolutions, despite the lack of a definition. The failure to define terrorism continues, however, to seriously impede the effectiveness of counterterrorism, its consistency with human rights law and international humanitarian law, and the legitimacy and legality of the Council’s exercise of its international security powers under the United Nations Charter.

Impaired Effectiveness

Firstly, from a practical standpoint, the inevitable divergence between national definitions impairs inter-state cooperation to “bring to justice” terrorists, as the Council requires. The “dual criminality” requirement of many extradition and mutual assistance laws and treaties may preclude cooperation between two states if their definitions of terrorism do not cover common ground. Impunity may accordingly result where a state is unable to extradite a person who may have committed a terrorist offense abroad under another state’s law, but not under its own. 

The same state may also be unable to prosecute the foreign offender in its own legal system. In the absence of an agreed international definition, one state is not required to criminalize the same “terrorist” conduct as another state by asserting extraterritorial quasi-“universal” jurisdiction over it. Instead, different domestic counterterrorism laws sail by like ships passing in the night. The legal differences are also fertile ground for political tensions where one state comes under pressure from another, or its allies, to assist the latter to enforce its terrorism laws.

Beyond the criminal law sphere, divergent national definitions equally impair cooperation across the spectrum of other measures required by Council resolutions. These include the duty on states to themselves refrain from supporting terrorism and to counter terrorist financing, prevent terrorism and support for it, prevent the movement of terrorists, address abuse of refugee status, and prevent and suppress the travel of “foreign terrorist fighters.” The same may be said of the “soft” counterterrorism agenda expressed through the UN General Assembly’s Global Counter-terrorism Strategy and the technical work of bodies such as the UN Office of Counter-Terrorism, UN Counter-Terrorism Centre, UN Office of Drugs and Crime, and through the 42 entities under the UN Global Counter-Terrorism Coordination Compact.

Secondly, although states may define terrorism for themselves, it is difficult for them to know whether there is any minimum or core “soft” concept of terrorism that they are nonetheless expected to enact. This matters for a bundle of reasons. States are subject to continuing international monitoring by the Counter-Terrorism Committee Executive Directorate (CTED) and the Financial Action Task Force (FATF) whose listing of high-risk jurisdictions can bring economic costs. Informal groups, such as the Global Counter-Terrorism Forum, also produce “soft” standards predicated on “terrorism,” which in turn influence Security Council and national practice. Other states may expect cooperation in respect of specific conduct. Technical assistance may also be premised on ‘soft’ definitions of terrorism.  

Stung by human rights criticisms, the Council belatedly offered a non-binding definition of terrorism in Resolution 1566. That definition has been welcomed on human rights grounds because it is so narrow. It cumulatively requires: (a) an intention to cause death or serious bodily injury or hostage taking, (b) an offense under one of the 19 existing “counterterrorism” conventions, and (c) a purpose (or “specific intent”) to provoke a state of terror in the public or a group of persons, or to intimidate a population, or to compel a government or international organization to do or to abstain from doing any act.

The advantages of that definition include that it links terrorism to the existing convention offenses, which are widely agreed upon, carefully negotiated through open and participatory treaty processes, and satisfy the principle of legality (including precision and foreseeability) in the definition of crimes. The addition of the personal violence and specific intent elements in Resolution 1566 also reduces the overbreadth of some convention offenses, which are not always “terroristic” but pursue wider regulatory objectives in fields such as aviation, maritime, or nuclear safety. Many of the conventions also capture not only “public” (that is, political, religious, or ideological) violence but also “private” violence, which may be more like ordinary crime than terrorism.

The obvious defect in this definition is that it would exclude many acts commonly regarded as “terrorism.” In part this is because the resolution covers only harm to people, not other targets of terrorism such as property, resources, infrastructure or utilities, communications, financial systems, the environment, or endangerment of public health and safety in general.

More pressingly, it is too narrow because it confines terrorism to the scope of the existing convention offenses. While these cover some common terrorist methods (particularly hostage taking and bombings), they were developed reactively and do not cover all forms of terrorism—or even the most common, such as attacks by small arms. Many are also limited to transnational not domestic terrorism, yet the Council also requires action on the latter.

These limitations are precisely why many states have enacted more general definitions of terrorism, which is also the aim of the Draft UN Convention for the Prevention and Suppression of International Terrorism, under negotiation since 2000 through the General Assembly. The draft convention, like the general part of the definition in the Terrorist Financing Convention 1999, inclusively encompasses terrorism “by any means.”

In any event, Resolution 1566 has not appreciably influenced national practice, and there is little evidence that the Council or CTED has sought to influence states to restrain or amend their definitions according to it. CTED’s Technical Guidance (2020) to states encourages enactment of the counterterrorism convention offenses, and acknowledges simply that other definitions of terrorism should comply with human rights law. However, the opaque nature of CTED assessments—most of the time, they are not publicly released—makes it difficult to know what precisely CTED expects of national definitions in practice.

Consistency with Human Rights and Humanitarian Law

The absence of definition has notoriously led to too many national definitions violating international human rights law. Particular concerns include infringements of the principle of legality, whereby definitions are not sufficiently clear or precise to enable people to foreseeably know the scope of their liability; as well discrimination and violations of political freedoms. These problems are most acute in relation to the many vague “preparatory” offenses which the Council requires states to enact, often compounding the vagueness in the predicate definition of “terrorism” itself. Excessive foreign laws may be yet another impediment to international cooperation where other states’ laws preclude complicity in rights violations. 

Admittedly, Council resolutions have tokenistically (and again belatedly) urged states to comply with human rights when implementing counterterrorism measures. It is also true that the burden is on states themselves to comply with human rights, including in defining terrorism. Nonetheless, the Council is negligent in allowing states relatively open slather on definition, when widespread abuse of terrorism laws is historically obvious and presently notorious, and when so many of its other measures are triggered by the definition. Promoting and encouraging respect for human rights is a core UN purpose. 

Council guidance has also been sorely lacking on the impact of definitions on international humanitarian law (IHL). A large part of the controversy about defining terrorism is precisely how to differentiate it from hostilities in armed conflict governed by IHL, including combat by non-state armed groups—even if ostensibly “terrorist” under counterterrorism law. Exclusions of various kinds are found in many counterterrorism conventions and regional law, as in the European Union. For many states, “war” is simply not the same as “terrorism,” even if some—but not all—tactics in war may justifiably be addressed by both IHL and counterterrorism.

In contrast, those national definitions of terrorism which criminalize all war fighting by armed groups—even if they respect IHL—undermine incentives for such groups to comply with IHL and impair prospects for peace settlements and post-conflict reconciliation. While the Council has abstractly urged states to comply with IHL, it has not confronted in any meaningful way how to properly address the interaction of the two legal regimes. 

Worse, certain terrorism offenses directly collide with the protections for medical and humanitarian personnel and activities under IHL, jeopardizing assistance to civilians, the wounded, and detainees. While the Council has encouraged (but not required) states to “take into account” such humanitarian imperatives, it has not unequivocally subordinated excessive counterterrorism laws (such as comprehensive “material support” offenses) to IHL.

Conclusion: Legitimacy and the Council’s Security Powers

The normative black hole at the center of the Council’s normative counterterrorism universe has not been an insurmountable obstacle to pragmatic lawmaking and cooperation since 2001. It has nonetheless impeded maximally effective and principled counterterrorism cooperation, led to frequent and relatively unconstrained human rights violations, and undermined policy and legal interests under IHL in armed conflict.

Since 2001 there have been debates about the Charter-based constitutionality of the Council’s quasi-legislative response to the generalized threat of any future terrorism, in contrast to its historical “policing” (not legislative) role in response to specific (not abstract) threats. In principle, it is arguable that responding to terrorism as a general category of threat, and requiring states to legislate prospectively in response, is a defensible exercise of Charter security powers, understood dynamically and in light of broad state acceptance. This is true even if other bodies or processes are undoubtedly better placed to make more legitimate law than the less transparent, selective club that is the Council—such as participatory, transparent, negotiated, consent-based multilateral treaty-making, through the universal General Assembly.

There are nonetheless other fundamentally troubling questions about the extent of this radical legal trajectory. How can the Council—credibly, and with a straight face—designate all “terrorism” as a threat to international peace and security, and require legal measures to be taken against it, without explaining what it is? How is all domestic (as opposed to transnational) terrorism a threat to international security, when its effects by definition are contained entirely within a single state? It may be acceptable to identify a general category of threat, but it can hardly be a valid exercise of Charter power if that threat comprises a black hole. The Council may be an expert in politics and security, but law requires certainty and precision—or it is not law at all, just politics and arbitrariness disguised as law.

This article is part of a series on the role of the UN system in preventing violent extremism and countering terrorism (PVE/CT), done in collaboration with the Brian Urquhart Center for Peace Operations.

Ben Saul is Challis Chair of International Law at the University of Sydney and Associate Fellow of Chatham House in London. He tweets at @profbensaul.