Defining Terrorism in International Law

By Ben Saul

Professor Ben Saul is Challis Chair of International Law at the University of Sydney, Australia and an Associate Fellow of the Royal Institute of International Affairs in London. He has taught at Harvard, Oxford, The Hague Academy of International Law and Italy, India, Nepal, and Cambodia, and been a visitor at the Max Planck Institute for International Law and and the Raoul Wallenberg Institute for Human Rights. He has published 20 books and over 100 refereed articles, including the books Defining Terrorism in International Law (2006), the Oxford Commentary on the International Covenant on Economic, Social and Cultural Rights (2014) (awarded a Certificate of Merit by the American Society of International Law), Research Handbook on International Law and Terrorism (2020), Oxford Guide to International Humanitarian Law (2020), and the Oxford Handbook on International Law in Asia and the Pacific (2019). Ben has advised United Nations entities, governments, parliaments, militaries, intelligence services, and NGOs; practiced in international courts; served on numerous professional bodies; and undertaken missions in over 35 countries. He has a doctorate from Oxford and honours degrees in Arts and Law from Sydney.

Published November/December 2021

1. Introduction

Despite the near-universal condemnation of ‘terrorism,’ the international community has repeatedly failed to define it as a legal category since the 1920s. The ordinary meaning of terrorism is simple: extreme fear. Translating that literal understanding into an agreed legal definition has, however, proved exceedingly difficult.

The challenge is ultimately not technical—anything can be positively defined by a treaty—but political. For some states, the emphasis should be on condemning state violence, while exempting violence in pursuit of self-determination or national liberation from foreign occupation. Other states prefer to focus on non-state violence, irrespective of motivation, including because state violence is already regulated by other rules (such as the prohibition on the use of force and intervention, international humanitarian law (IHL), and human rights law). Technical issues have also still been challenging, including how to ensure a definition is not over- or under-inclusive and does not violate human rights.

At the international level, there is now a basic legal consensus that terrorism is criminal violence intended to intimidate a population or coerce a government or international organisation; some national laws add a further specific intention to advance a political, religious, or ideological cause. There remain strong political disagreements, however, on the extent to which state and self-determination violence should be included, and on the relationship between counter-terrorism law and IHL (which already regulates all violence in armed conflict). As a result, a conceptual impasse continues, even if international legal agreement has been edging closer.

In the absence of agreement, from the 1960s to the 1990s most states responded to terrorism by applying a mixture of ordinary domestic offences and special public order or national security offences. These were supplemented in cross-border cases by the domestic implementation of transnational ‘sectoral’ treaty offences (which criminalize common terrorist methods, such as hijacking or hostage taking, but do not define terrorist offences as such); a few of the later treaties also depoliticize their offences in extradition (such as terrorist bombings (1997) and terrorist financing (1999)).

It was only after the terrorist attacks on the United States of September 11, 2001 (‘9/11’) that most states began enacting ‘terrorism’ offences, prompted by the UN Security Council’s obligation on states, under its resolution 1373 (2001), to criminalize terrorist acts in domestic law—without the Council providing a definition. For the first time under international law, the notion of ‘terrorism’ now has operative legal significance. While terrorist offences are now very common in national laws, definitions of terrorism remain startlingly diverse. This article examines the main international attempts to define terrorism, including current efforts since 1999 at the United Nations to negotiate a Draft Comprehensive Terrorism Convention and practice arising from the Security Council’s response.

International definition of terrorism matters principally in delineating criminal offences, as well as closely related areas of law (including law enforcement powers, criminal procedure, and extradition and mutual legal assistance). However, the scope of offences also has implications for international humanitarian law and compliance with human rights law. Its relevance is less obvious to the law on the use of force, whose operative legal concepts (such as armed attack and self-defence) do not hinge on whether an actor is ‘terrorist’ or not. Definition can, however, matter in other areas, such as international cooperation on the prevention of terrorism and violent extremism, technical assistance, and United Nations programmes, as well as in shaping national approaches to, for instance, counter-terrorism sanctions and the listing (and freezing or confiscation of assets) of individuals and entities.

2. Terrorism and the Law Before 1945

The term entered into political discourse in the late eighteenth century to describe the Jacobin reign of (state) terror during the French revolution.[1] Its usage has since vacillated between stigmatizing various kinds of state and non-state violence, depending on the disapprover’s perspective. While there is a long and contested history of moral and political disapproval of ‘terrorism,’ it is only recently that concepts of terrorism have taken legal form.

From the mid-nineteenth century, the problem of what is often now regarded as terrorism was framed quite differently in national law. Political violence was prosecuted as ordinary crime or offences against public order or state security. Where perpetrators of political violence fled across national borders to escape retribution or justice, victim states often demanded their surrender. However, highly variable national extradition laws and bilateral treaties often stymied cooperation in such cases, whether because of the ‘double criminality’ requirement (the conduct must be an offence in both jurisdictions) or the ‘political offence exception’ (precluding extradition where offences are ‘political’) and the related protection of political asylum.[2] This resulted in inter-state tensions, as well as impunity for all offenders—including those who indiscriminately killed or injured civilians.

Modern transnational efforts to manage ‘terrorism’ stemmed from efforts to address these legal barriers to cooperation. Starting in 1856, the Belgian ‘attentat’ (attempt) clause regarded assassinations of heads of state or government as extraditable—even if they were politically motivated—and the idea spread to other western European states and to the United States. In the 1890s, the rise of anarchist violence in Europe and other continents—seen as ‘asocial’ and the enemy of all political systems—stimulated efforts to depoliticize it in extradition, evident in a resolution of the International Law Institute in 1892 and at the Rome Anti-Anarchist Conference of 22 states in 1898. (A few national laws in the mid-1890s (in Spain, Italy, and Switzerland) also criminalized ‘terrorism’ to counter terrorism.)

These developments still did not address other kinds of more controversial violence which proliferated in the nineteenth century—political, nationalist, separatist, and socialist—and which predominated after the First World War (when anarchism declined). However, multilateral agreement to narrow the scope of the political offence exception—beyond targeted assassinations or anarchism—proved too difficult before the Second World War. The idea of ‘terrorism’ nonetheless increasingly became a focus of efforts towards reform.

Elite civil society organizations sought to define terrorism from the late 1920s to the mid-1930s, for the purpose of harmonized codification in national criminal law and depoliticization in extradition. The jurists of the International Association of Penal Law and its off-shoot, International Bureau for the Unification of Criminal Law, debated whether political terrorism was distinguishable from anarchist ‘social crime.’ In 1935 they defined terrorism as attacking internationally protected persons to create a ‘state of terror’ which impedes government or disturbs international relations.[3] Yet, they could not agree to depoliticize terrorism extradition—including because of liberal concerns about rising authoritarianism in the 1930s. The proposal did not influence national laws and was overtaken by a seismic event at the League of Nations in 1934.

The most important international effort to legally address ‘terrorism’ in this period followed the 1934 assassination in France of King Alexander of Yugoslavia and the French Foreign Minister, Louis Barthou, by a Macedonian separatist. The Italian courts refused to extradite the fugitives on the basis of the political offence exception. To avert possible conflict, from 1934 to 1937 the League of Nations drafted an international Convention to repress the crime of terrorism, and another treaty establishing an international criminal court to prosecute it.[4]

The 1937 Convention for the Prevention and Punishment of Terrorism required States to criminalize terrorist offences. Article 1(2) defined ‘acts of terrorism’ as ‘criminal acts directed against a [foreign] State and intended or calculated to create a state of terror in the minds of particular persons, or a group of persons or the general public.’ Article 2 enumerated the physical acts which States must criminalize, including crimes against persons and property, weapons offences, and ancillary offences. Consequently, terrorism is defined by the intended aim (a state of terror), the ultimate target (a State), and the prohibited means used, but proposals to define terrorism as a means to a political end were not accepted.[5] ‘Acts of terrorism’ was defined circularly by reference to ‘a state of terror,’ despite objections that the phrase was ambiguous and open to abuse.

The Convention’s extradition provisions still did not exclude terrorism from the political offence exception. In a climate of mounting authoritarianism, many states were reluctant to confine their sovereign discretion in extradition matters, including the scope of political offences, and were at pains to protect asylum from degradation. With the coming of the Second World War and the demise of the League, the treaty never entered into force. As a result, the predominant approach in national laws remained that terrorist violence was prosecuted as ordinary, political or security crimes, but not as terrorism. Further, in transnational cases, extradition remained subject to varied approaches to the political offence exception. The League definition nonetheless influenced later international legal debates.

3.1. Codification of International Crimes

After the Second World War, the concept of terrorism resurfaced in episodic efforts by the International Law Commission (ILC) to codify international crimes between 1954 and 1998. An important shift occurred towards regarding terrorism as a state-sponsored phenomenon rather than violence by non-state actors. In 1954 the ILC invoked, but did not define, ‘terrorism’ as criminal aggression by one state against another.[6] Later in 1991 it proposed an offence of international terrorism by one state against another, defined as:

undertaking, organizing, assisting, financing, encouraging or tolerating acts against another State directed at persons or property and of such a nature as to create a state of terror in the minds of public figures, groups of persons or the general public.[7]

Its revised 1995 draft added that acts must be committed ‘in order to compel’ the victim State ‘to grant advantages or to act in a specific way.’[8] The final ILC draft code of international crimes was approved in 1996 but no longer included a terrorism offence.[9] The General Assembly drew the 1996 ILC Draft Code to the attention of the Preparatory Committee on the Establishment of an International Criminal Court (ICC).[10] Ultimately, Article 5 of the 1998 Draft Rome Statute, presented to the 1998 Rome Diplomatic Conference,[11] revived separate ‘crimes of terrorism,’ comprising three distinct offences. The first offence was:

Undertaking, organizing, sponsoring, ordering, facilitating, financing, encouraging or tolerating acts of violence against another State directed at persons or property and of such a nature as to create terror, fear or insecurity in the minds of public figures, groups of persons, the general public or populations, for whatever considerations and purposes of a political, philosophical, ideological, racial, ethnic, religious or such other nature that may be invoked to justify them….

This first offence resembles the 1991 ILC draft and was not limited to armed conflict (as was the 1996 ILC draft). It also shares elements of the 1937 League definition and a later 1994 General Assembly working definition of terrorism.[12] The second offence comprised any offence in six sectoral anti-terrorism treaties (discussed below). The third offence involved ‘the use of firearms, weapons, explosives and dangerous substances when used as a means to perpetrate indiscriminate violence involving death or serious bodily injury to persons or groups or persons or populations or serious damage to property.’

At the Rome Conference, 34 states spoke in favor of including terrorism, including because it shocked the conscience of humanity; had grave consequences for human suffering and property damage; occurred increasingly frequently and on a larger scale; and threatened peace and security.[13] The option of referring terrorism to the ICC was also intended to avoid jurisdictional disputes between states and empower the Security Council to resolve it.

Ultimately, terrorism was not included in the 1998 Rome Statute. A conference resolution ‘regretted’ that ‘despite widespread international condemnation of terrorism, no generally acceptable definition… could be agreed upon.’[14] The issue of national liberation violence was contentious, and some feared terrorism would politicize the ICC.[15] Pragmatically, some states felt that terrorism was better suited to national prosecution (not serious enough for international prosecution) or that investigative—not legal—difficulties were decisive.[16] Of the 23 states that spoke against including terrorism, many agreed that it was a serious crime but preferred to defer its inclusion until it was defined more clearly. The omission of terrorism is significant in signaling that the international community in 1998 did not view it as an agreed international crime. As of 2015, terrorism is yet to be included in the ICC’s jurisdiction, despite attempts in the early 2000s by the Netherlands amend the ICC Statute to include it.

3.2. UN General Assembly

While the ILC is a subsidiary organ of the UN General Assembly, the General Assembly itself separately turned its attention to the problem of terrorism throughout the 1970s. Disagreement was acrimonious after an attack by Palestinian militants on Israeli athletes at the Munich Olympics in 1972. In UN debates between 1973 and 1979, focused through an Ad Hoc Committee, states were unable to agree on a definition of terrorism, the causes of it, or measures to address it. Disagreement was particularly sharp on liberation violence and state terrorism, while socialist states also accused the western powers of imperialistic and capitalistic violence.

From the 1980s however, more consensus slowly developed, and accelerated in the 1990s after the end of the Cold War. A breakthrough came with the 1994 Declaration on Measures against International Terrorism (A/RES/49/60), which condemns terrorism as ‘[c]riminal acts intended or calculated to provoke a state of terror in the general public, a group of persons or particular persons for political purposes.’[17] It adds that such acts ‘are in any circumstance unjustifiable, whatever the considerations of a political, philosophical, ideological, racial, ethnic, religious or any other nature that may be invoked to justify them.’ Regular subsequent resolutions affirmed this approach. The General Assembly and Security Council have further identified terrorism as a threat to international security, human rights, and stable governance.[18]

Despite the growing political consensus, many states have continued to differentiate self-determination violence from terrorism and insist on the need for a legal (as opposed to the above political) definition—particularly the 118 states of the Non-Aligned Movement and the (many overlapping) 56 states of the Organisation of Islamic Cooperation (OIC). While the definition did not establish legal liability or crime of terrorism, or of generate customary law, it indicated the world’s basic political conception of terrorism.

3.3. Sectoral Counter-Terrorism Conventions

In the absence of a generally agreed international legal definition of terrorism after 1945, the international community nonetheless was able to take pragmatic steps to counter terrorism. Transnational terrorist acts increased from the 1960s, often perpetrated by liberation movements resisting colonial powers, and reflected in acts such as aircraft hijacking, hostage taking against diplomats or attacks on embassies, and other political violence. The international community responded incrementally by adopting numerous ‘sectoral’ treaties since the 1960s—now numbering 19—addressing common methods of terrorist violence (such as hijacking, hostage taking, endangering maritime facilities and so on).[19] Most of the treaties avoid referring to ‘terrorism,’ with the exception of three recent treaties (since 1997) on terrorist bombings (1997), terrorist financing (1999), and nuclear terrorism (2005).

The treaties typically require states to criminalise certain conduct, establish extraterritorial jurisdiction, and cooperate by prosecuting or extraditing suspects (the aut dedere aut judicare principle). A few recent treaties require states to regard the offences as non-political for the purposes of extradition, but most treaties do not. None of the treaties establishes a general crime of terrorism. although the Terrorist Financing Convention comes closest in providing a general definition for the limited purpose of criminalising terrorist financing:

Any other act intended to cause death or serious bodily injury to a civilian, or to any other person not taking an active part in the hostilities in a situation of armed conflict, when the purpose of such act, by its nature or context, is to intimidate a population, or to compel a government or an international organization to do or to abstain from doing any act.[20]

This pragmatic approach has enabled the repression of much terrorism while side-stepping the irreconcilable problem of definition. The result has been functional transnational cooperation, even if there remain regulatory gaps because of the reactive, ad hoc nature of treaty-making. For example, some of the most common contemporary methods, such as attacks by small arms (as in the Mumbai attacks in 2008) are not criminalized by the treaties.

3.4. UN Draft Comprehensive Convention

The closest the UN has come to generally criminalizing terrorism is in the ongoing negotiation of a Draft Comprehensive Terrorism Convention since 1999, based on an Indian proposal. Agreement was reached on most of the 27 articles by 2002,[21] including definition, spurred on by the 9/11 attacks. Negotiations stalled, however, over the scope of application to certain non-state and state violence in armed conflict, as well as the activities of state militaries in peacetime. Despite regular meetings, as of 2021 there is still no agreement.

Draft Article 2(1)[22] proposes an offence if a person ‘unlawfully and intentionally’ causes: ‘[d]eath or serious bodily injury to any person’; ‘[s]erious damage to public or private property’; or ‘[d]amage to property, places, facilities, or systems… resulting or likely to result in major economic loss.’[23] The purpose (or specific intent) of such conduct, ‘by its nature or context,’ must be ‘to intimidate a population, or to compel a Government or an international organization to do or abstain from doing any act.’ The treaty would exclude its offences from the political offence exception to extradition.

The definition does not require proof of a political, religious, or ideological purpose, as in some definitions of terrorism in some common law countries. There is also no ‘democratic protest’ exception as in some national laws, which exclude acts of advocacy, protest, dissent or industrial action, which are not intended to cause death, serious bodily harm, or serious risk to public health or safety.[24] Such exceptions prevent criminalizing as ‘terrorism’ minor harms (property damage or public order offences) in the course of the traditions of direct democratic action. Like most national laws, however, the Draft Convention does not include exemptions for other ‘just causes’ such as rebellion against tyrannical regimes; it is equally ‘terrorism’ to assassinate Hitler as it is to attack the head of a rights-respecting democracy.

The key unresolved controversy concerns exceptions to the offences, which is really a debate about what is or is not ‘terrorism.’ The Draft Convention proposes to follow the Terrorist Bombings Convention in excluding the activities of ‘armed forces’ in ‘armed conflict,’ which are governed by IHL, and as those term as understood under IHL. The exclusion clause partly defers to IHL as the special law (lex specialis) adapted to regulating conflicts, and thus avoids criminalizing as terrorism violence by fighters. IHL already criminalises deliberate attacks on civilians or certain unlawful attacks on the military in armed conflict. Exempting the above conduct from the Convention crime of terrorism would not therefore confer impunity but leave liability to war crimes law. However, the exclusion clause is arguably too broad since it excludes all activities of armed forces—not only those which comply with IHL—including attacks on civilians. Since counter-terrorism laws often involve stronger investigative powers and extended modes of criminal liability, dually criminalizing unlawful acts under IHL as terrorism could bring advantages for civilian protection.

In the drafting of the Comprehensive Convention, the OIC has sought to amend the proposed exclusion clause. It OIC suggests excluding the activities of the ‘parties’ not the ‘armed forces,’[25] including in situations of ‘foreign occupation.’[26] This stems from uncertainty about whether ‘armed forces’ includes non-state armed groups or only means state militaries, and whether armed conflict includes occupation. On the latter, it is clear under IHL that occupation is a form of international armed conflict.[27] On the former point, while some states have suggested that armed forces means only state forces, this is not correct under IHL: common article 3, which applies equally to non-international armed conflicts between states and non-state actors, explicitly refers to ‘armed forces.’[28] As such, the OIC proposals are both unnecessary, although any lingering ambiguity about ‘armed forces’ could be cured by simply defining them in the Convention itself to explicitly include non-state forces.

The OIC proposal to include the ‘parties’ may, however, also aim to exclude the prosecution of civilians taking a direct part in hostilities under IHL, who are not integrated members of non-state ‘armed forces’ performing a continuous combat function. Only the latter, as ‘fighters,’ would normally be regarded as part of the ‘armed forces’ and thus exempt from the Convention. If this is the case, it would further reduce liability for terrorism, excluding individuals who are not part of (potentially) disciplined fighting force under IHL, and who may therefore be more likely to commit violations against civilians.

The Draft Convention additionally proposes to follow the Terrorist Bombings Convention in excluding the activities of state military forces exercising their official duties in peacetime, which are ‘governed’ by international law. The OIC proposes to exclude them only if their activities are ‘in conformity’ with international law. Official duties in peacetime include, for instance, law enforcement, evacuation operations, peace operations, UN operations, or humanitarian relief. The OIC feels that the convention should cover excessive state violence in peacetime as ‘terrorism,’ notwithstanding the application of existing international laws. Presently, state violations of international law (including the law on the use of force and on human rights), does not always entail criminal liability, whereas the Convention would.

3.5. The War Crime of Terrorism

One of the few branches of international law that has expressly embraced the concept of terrorism is IHL, which as mentioned applies only in armed conflict (international—between states, or non-international law—between states and non-state armed groups, or between such groups). Various IHL treaties prohibit terrorism,[29] in reaction to the intimidation of civilians by the fascist powers in the Second World War.[30] In 2003 the International Criminal Tribunal for the former Yugoslavia recognised, in the Galić case, that violations of these treaty provisions may constitute the war crime of intending to spread terror among the civilian population.[31] Terror was defined simply as ‘extreme fear.’[32]

On the facts in Galić, the war crime of terror was found to have been committed by a campaign of sniping and shelling of civilians in the besieged city of Sarajevo, as a result of ‘the nature of the civilian activities targeted, the manner in which the attacks on civilians were carried out and the timing and duration of the attacks on civilians.’[33] The Special Court for Sierra Leone has also entered convictions for the war crime of terrorism,[34] committed by acts such as amputations and mutilations of civilians. The war crime of terror is distinct from the peace-time conceptions of terrorism mentioned earlier,[35] namely violence to compel a government or international organisation to do or refrain from doing something.[36]

3.6. Counter-Terrorism Laws of Regional Organisations

In the absence of universal agreement, greater progress has been made at the regional level. While some regional conventions take a ‘sectoral’ approach like the international treaties, others generically define terrorism, including those of the League of Arab states (1998), OIC (1999), African Union (1999), and the Shanghai Cooperation Organisation (2001).[37] While not a treaty, also relevant is the European Union’s Directive on Combating Terrorism 2017,[38] which requires approximation of ‘terrorist offences’ in domestic law in EU member states.

The regional conventions have given rise to the human rights concerns because many of their definitions of terrorism are drafted loosely and fail to satisfy the principle of legality. Some of them reclassify as terrorism ordinary crimes or public order offences,[39] or even insurrection.[40] Some criminalise conduct infringing vague values such as the ‘stability, territorial integrity, political unity or sovereignty’ of States or imperiling the ‘honour’ or ‘freedoms’ of individuals.[41] Some safeguard ambiguous objects, such as harm to a ‘national resource’[42] or ‘environmental or cultural heritage.’[43] One inter-mingles terrorism with ‘separatism’ or ‘extremism.’[44] The EU includes an ill-defined motive element of ‘seriously destabilising or destroying the fundamental political, constitutional, economic or social structures of a country or an international organization.’[45]

The elimination of the political offence exception in the context of such wide definitions of terrorism is especially problematic, since it substantially curtails the freedom of populations to resort to domestic political resistance to violent, oppressive governments. Other states in region become legally obliged to suppress resistance, often even where such movements limit violence to IHL-compliant attacks on military objectives. At the other extreme, three conventions (OIC, Arab, and African) ‘carve out’ acts by liberation movements in pursuit of self-determination, problematically implying that any means may be justified for a just cause.

4. Terrorism After September 11, 2001

4.1. UN Security Council Resolution 1373 (2001)

Prior to 9/11, the UN Security Council sporadically condemned specific terrorist acts, and created a sanctions regime targeting particular members of Al Qaeda and the Taliban in Afghanistan. Its approach changed radically after 9/11. By resolution 1373 (2001), adopted under Chapter VII of the UN Charter, the UN Security Council directed all States to criminalize terrorism in domestic law (as well as universalising the offences in the then scantly ratified Terrorist Financing Convention).

Resolution 1373 did not define terrorism for the purpose of national criminalization, instead allowing states to unilaterally define it in implementation. This not only resulted in divergent definitions, but certain states also defined terrorism to suit their own political purposes or to undermine human rights. 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 as 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.

From a counter-terrorism standpoint too, the lack of definition is counter-productive, since the 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 offence 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 international definition, it is not required to criminalize the same ‘terrorist’ conduct as another state by asserting extraterritorial quasi-‘universal’ jurisdiction. 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 others to enforce their 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 the 42 entities under the UN Global Counter-Terrorism Coordination Compact.

Council guidance has also been sorely lacking on the impact of definitions on IHL. Some national terrorism offences (as in some EU states) contain exclusions (sometimes based on the Terrorism Bombings Convention model, discussed above, but extended to all terrorism offences). In contrast, other national definitions of terrorism (as in the UK or Australia) criminalize all war fighting by armed groups (namely, as political violence to coerce a foreign state)—even if they respect IHL. This is so even if the attack solely aims at a military target, does not cause disproportionate civilian casualties, and is not perfidious or does not otherwise use prohibited means or methods of warfare. In R v Gul [2013], the UK Supreme Court found that, while various counter-terrorism treaties exempt aspects of armed conflict from their scope, international law does not prohibit national law from extending domestic terrorism offences to apply in armed conflict.[46]

This approach does, however, undermine incentives for armed groups to comply with IHL, since there is no longer any difference in legal consequences between attacking civilians or the military. 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. It can also impair prospects for peace negotiations and post-conflict reconciliation. While the Council has encouraged (but not required) states to ‘take into account’ such humanitarian imperatives, and abstractly urged states to comply with IHL in general, it has not properly address the full interaction between the regimes.

4.2. Security Council Resolution 1566 (2004)

Faced with a backlash from human rights bodies and civil society, in resolution 1566 (2004) the Security Council eventually signalled its conception of terrorism as:

…criminal acts, including against civilians, committed with the intent to cause death or serious bodily injury, or taking of hostages, with the purpose to provoke a state of terror in the general public or in a group of persons or particular persons, intimidate a population or compel a government or an international organization to do or to abstain from doing any act, which constitute offences within the scope of and as defined in the international conventions and protocols relating to terrorism…[47]

The cumulative definition reclassifies harmful acts as ‘terrorism’ only where they are designed to terrorize, intimidate or compel, and already constitute a sectoral treaty offence. The UN Special Rapporteur on human rights and terrorism was satisfied that it reflects a narrow and rights-respecting concept of terrorism. 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.[48] The addition of the personal violence and specific intent elements also reduce the overbreadth of some terrorism convention offenses, which are not always ‘terroristic’ but pursue wider regulatory objectives in fields such 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. The definition also does not articulate the desirable relationship between terrorism and IHL (other than by renvoi to the sectoral conventions, only some of which address such relationship), and by its silence implicitly envisages the co-application of terrorism offences in armed conflict. These limitations are precisely why many states have enacted more general definitions. Security Council Resolution 1566 is also only a working definition which does not require states to conform their laws to it and has had little apparent influence in practice.

4.3. Legitimacy of Council Measures

Since 2001 there have been debates about the Charter-based constitutionality of the Council’s quasi-legislative response (in resolution 1373) 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.

4.4. Terrorism as a Customary International Law Crime?

While neither treaties nor the Security Council has defined terrorism, in 2011 the Appeals Chamber of the hybrid UN Special Tribunal for Lebanon, established to prosecute terrorist bombings in Lebanon in 2005, purported to identify an extant customary international crime of terrorism in peacetime, and applied it in interpreting domestic terrorism offences under Lebanese law. The crime consists of three elements:

(i) the perpetration of a criminal act (such as murder, kidnapping, hostage-taking, arson, and so on), or threatening such an act; (ii) the intent to spread fear among the population (which would generally entail the creation of public danger) or directly or indirectly coerce a national or international authority to take some action, or to refrain from taking it; (iii) when the act involves a transnational element.[49]

The requirement of a transnational element[50] rules out purely domestic terrorism. While the Tribunal recognised only peace-time terrorism as a crime, it indicated that ‘a broader norm that would outlaw terrorist acts during times of armed conflict may also be emerging.’[51] The generic elements of the offence are suggestive of the definition in the 1999 Terrorist Financing Convention and the UN Draft Comprehensive Convention. It is broader than the narrow, rights-respecting definition offered by the Security Council in resolution 1566. The Appeals Chamber acknowledged that a political or other motive element (as found in some common law definitions, mentioned above) would narrow the definition, prevent its over-expansive application, and further the principle of legality, but felt it was not yet part of the customary law definition, even if it might become so in future.[52]

In reaching its conclusion, the Appeals Chamber found that the 37 national terrorism laws it cited are broadly ‘concordant’ and evince ‘a widespread stand on and a shared view of terrorism.’[53] In particular, it was stated that ‘[e]lements common across national legislation defining terrorism include the use of criminal acts to terrorise or intimidate populations, to coerce government authorities, or to disrupt or destabilise social or political structures.’[54]

National laws can certainly provide evidence of state practice in the formation of customary international law. However, the Appeals Chambers’ conclusion is dubious. It conflates national laws addressing national and international terrorism and of varying jurisdictional reach. It conflates criminal definitions with non-criminal definitions. Further, close inspection of the national laws cited shows that many of the laws do not converge at all, but represent fundamentally different conceptions of terrorism, including civil war and sectarian strife (Iraq), public disorder (Egypt), constitutional subversion (Peru), harm to international relations, sovereignty or territorial integrity (Uzbekistan), or violation of honour (Saudi Arabia). Looking beyond the limited number of 37 ‘best example’ laws cited by the Appeals Chamber, it is clear that legal approaches to terrorism in the bulk of national legal systems—including the 160 states not mentioned—are even more divergent.

While the Appeals Chamber also invokes Security Council resolution 1566,[55] it does not support the Appeals Chamber’s definition precisely because the resolution is narrowly pegged to sectoral treaty offences, whereas the Appeals Chambers’ definition is not. The other sources relied on—including UN General Assembly resolutions, international and regional treaties, and national judicial decisions—also do not sustain the conclusion that terrorism is a customary international crime.[56] Recent authoritative national court decisions have not accepted the view that there is an agreed international definition.[57]

5. Conclusion

At one level, a legal category of terrorism may be seen as unnecessary: terrorist violence can usually be prosecuted as ordinary crime or national security offences. At the same time, however, it can add special definitional elements which differentiate it from other crimes and thereby signal the international community’s condemnation of (for instance) instrumental political or religious violence intended to intimidate a population or coerce a government. Pragmatically, it can trigger special powers and procedures, and preventive offences and measures. It can also facilitate transnational cooperation and extradition and plug gaps in the existing ad hoc sectoral counter-terrorism treaties.

Legal concepts of terrorism bring risks too. Excessively wide or loose concepts of terrorism can seriously jeopardize internationally protected human rights. The absolutist politics of state survival and national security frequently taint the drafting and use of terrorism laws. Often the wide special powers and offences that attach to a definition, and the absence or degradation of ordinary safeguards, are even more dangerous to human rights.

Terrorism laws make most sense when protecting a democracy from violent adversaries, and less sense when they shield authoritarian states from those who rightly resist it. International agreement on terrorism in a diverse community of states is so difficult for this reason. Most minimally agree that the instrumental political killing of civilians in peacetime is terrorism. Beyond that, ‘terrorism’ remains a contested terrain—and there remains an urgent need for the international community to precisely define it in a rights-respecting way.

[1] Ben Saul, Defining Terrorism in International Law (Oxford University Press, 2006), 1.

[2] Christine Van den Wijngaert, The Political Offence Exception to Extradition (Kluwer, 1980), 191.

[3] CB Ingwersen and others (eds), VIe Conférence internationale pour l'unification du droit pénal: actes de la Conférence: Copenhague, 31 août-3 septembre 1935 (Pedone 1938).

[4] League of Nations Convention for the Prevention and Punishment of Terrorism (adopted 16 November 1937, never entered into force) and League of Nations Convention for the Creation of an International Criminal Court (adopted 16 November 1937, never entered into force), (1938) League of Nations Official Journal 19; Ben Saul, ‘The Legal Response of the League of Nations to Terrorism (2006) 4 Journal of International Criminal Justice 78.

[5] League of Nations (Committee on the International Repression of Terrorism), ‘Legislation regarding Political Terrorist Crimes: Study by T Givenovitch,” Geneva, 3 May 1935, LoN Doc CRT.9.

[6] ILC, Draft Code of Offences against the Peace and Security of Mankind (Part I), in ILC, Sixth Session Report (3 June–28 July 1954), UN Doc A/2693, article 2(6).

[7] ILC, Draft Code of Offences against the Peace and Security of Mankind 1991, in (1990) ILC Year Book 336, article 24.

[8] ILC, Report on its 47th Session (2 May-21 July 1995), UN Doc A/50/10, 58.

[9] ILC, Report on its 48th Session (6 May-26 July 1996), UN Doc A/51/10, paras. 46, 48.

[10] UN General Assembly resolution 51/160 (1996).

[11] Draft Rome Statute 1998, in Official Records of the UN Diplomatic Conference of Plenipotentiaries on an ICC, Rome, 15 June–17 July 1998, UN Doc A/CONF.183/13, article 5.

[12] UN General Assembly resolution 49/60 (1994), annexed Declaration on Measures to Eliminate International Terrorism.

[13] ICC Preparatory Committee, Summary of Proceedings, 25 March–12 April 1996, UN Doc A/AC.249/1 (7 May 1996), para. 66.

[14] UN Diplomatic Conference of Plenipotentiaries on an International Criminal Court, Final Act, 17 July 1998, UN Doc A/Conf.183/10, annex, Resolution E.

[15] ICC Preparatory Committee, above note 13, para. 67; Neil Boister, ‘The Exclusion of Treaty Crimes from the Jurisdiction of the Proposed International Criminal Court: Law, Pragmatism, Politics’ (1998) 3 Journal of Armed Conflict Law 27.

[16] Mahnoush Arsanjani, ‘The 1998 Rome Statute of the International Criminal Court’ (1999) 93 American Journal of International Law 22, 29; David Scheffer, ‘Developments at Rome Treaty Conference,” Testimony of US Ambassador at Large for War Crimes Issues and Head of US Delegation to the Rome Conference, US Senate Foreign Relations Committee, Washington DC, 23 July 1998.

[17] UN General Assembly resolution 49/60 (1994), para. 3. See also United Nations, Declaration on Measures to Eliminate International Terrorism and the 1996 Supplementary Declaration Thereto, 1994, Audiovisual Library of International Law,

[18] See Saul, above note 1, ch 2.

[19] For discussion of particular conventions see Ben Saul (ed), Research Handbook on International Law and Terrorism (Edward Elgar, 2014).

[20] Terrorist Financing Convention 1999, article 2(1)(b).

[21] UN General Assembly (57th Session) (Sixth Committee), Measures to Eliminate International Terrorism: Working Group Report, 16 October 2002, UN Doc. A/C.6/57/L.9, annex II, 7-8.

[22] UN General Assembly Official Records (68th Session), Supplement No. 3: Report of the Ad Hoc Committee established by General Assembly Resolution 51/210, 17 December 1996, 16th session (8–12 April 2013), UN Doc A/68/37, 6.

[23] UN General Assembly (56th Session) (Sixth Committee), Measures to Eliminate International Terrorism: Working Group Report, 29 October 2001, UN Doc A/C.6/56/L.9, annex I, 16.

[24] Canadian Criminal Code, s. 83.01(1)(E); Australian Criminal Code, s. 100.1(3); Terrorism Suppression Act 2002 (New Zealand), s. 5(5).

[25] UN General Assembly Sixth Committee, Report of the Working Group on Measures to Eliminate International Terrorism, 8 October 2004, UN Doc. A/C.6/59/L.10.

[26] UN General Assembly Ad Hoc Committee Report (2002), UN Doc A/57/37, annex IV, 17.

[27] Geneva Conventions, common art 2.

[28] Ben Saul, ‘From Conflict to Complementarity: Reconciling International Counter-terrorism Law and International Humanitarian Law (2021) International Review of the Red Cross (forthcoming).

[29] Fourth Geneva Convention 1949, article 33(1); Protocol I Additional to the Geneva Conventions of 1949, article 51(2); Protocol II Additional to the Geneva Conventions of 1949, articles 4(2)(d) and 13(2).

[30] Ben Saul, ‘Crimes and Prohibitions of “Terror” and “Terrorism” in Armed Conflict: 1919-2005’ (2005) 4 Journal of the International Law of Peace and Armed Conflict 264.

[31] Prosecutor v Galić, ICTY-98-29-T, Trial Chamber, Judgement, 5 December 2003, para. 138; Prosecutor v Galić ,IT-98-29-A, Appeals Chamber, Judgement, 30 November 2006.

[32] Galić (2003), ibid, para. 137.

[33] Ibid, paras. 592, 596-597.

[34] Prosecutor v Brima et al., SCSL-04-16-T, Trial Chamber, Judgement, 20 June 2007, paras. 662, 666; Prosecutor v Taylor, SCSL-03-1-T, Trial Chamber, Judgement, 26 April 2012, para. 112.

[35] Taylor, ibid, paras. 408-410.

[36] Saul, above note 1, chs 3-4.

[37] Arab League Convention on the Suppression of Terrorism 1999; OIC Convention on Combating International Terrorism 1999.

[38] Directive (EU) 2017/541 of the European Parliament and of the Council of 15 March 2017 on combating terrorism and replacing Council Framework Decision 2002/475/JHA and amending Council Decision 2005/671/JHA.

[39] Arab League Convention, above note 37, article 1(2); OIC Convention, above note 37, article 1(2).

[40] Organisation of African Unity (OAU) Convention on the Prevention and Combating of Terrorism 1999, article 1(3).

[41] OIC Convention, above note 37, article 1(2).

[42] Arab League Convention, above note 37, article 1(2); OIC Convention, above note 37, article 1(2).

[43] OAU Convention, above note 40, article 1(3).

[44] Shanghai Cooperation Organisation Convention on Combating Terrorism, Separatism and Extremism 2003, article 1.

[45] EU Directive 2017, above note 38, article 3(2)(c).

[46] R v Gul (Appellant) [2013] UKSC 64.

[47] UN Security Council resolution 1566 (2004), para. 3.

[48] See eg UN Special Rapporteur (Martin Scheinin), Report on the Promotion and Protection of Human rights and Fundamental Freedoms while Countering Terrorism, E/CN.4/2006/98 (28 December 2005), para. 42.

[49] UN Special Tribunal for Lebanon (Appeals Chamber), Interlocutory Decision on the Applicable Law: Terrorism, Conspiracy, Homicide, Perpetration, Cumulative Charging, STL-11-01/I, 16 February 2011, para. 85.

[50] Ibid, para. 90.

[51] Ibid, paras. 107-9.

[52] Ibid, para. 106.

[53] Ibid, para. 92.

[54] Ibid, para. 93.

[55] Ibid, para. 88.

[56] Ben Saul, ‘Legislating from A Radical Hague: The UN Special Tribunal for Lebanon Invents an International Crime of Transnational Terrorism’ (2011) 24 Leiden Journal of International Law 677.

[57] R v Gul (Appellant) [2013] UKSC 64, para. 44; Al-Sirri v Secretary of State for the Home Department [2013] 1 AC 745, para. 37.