The Prosecution of Terrorism as a War Crime
Despite the persistent condemnation of terrorism by States and international organizations, and the frequent prosecution of terrorism as an offence under domestic law, the prosecution of terrorism as a war crime is a relatively rare occurrence. There is no consensus even that terrorism is indeed a war crime under customary international law, despite certain high-profile convictions in international criminal tribunals.
This post summarizes the history of the prohibition of terrorism under the law of armed conflict (LOAC) and traces its transformation into not only a customary rule but also a purported customary war crime. It begins with a brief discussion of the regulation of terrorism under the global sectoral treaties that are primarily applicable in peacetime, before turning to the specifics of LOAC under the 1949 Geneva Conventions and the two 1977 Additional Protocols.
Terrorism under Counterterrorism Law
The origin of a global treaty prohibition is found in the League of Nations and its abortive 1937 Convention for the Prevention and Punishment of Terrorism. Article 1(2) of the Convention, which never entered into force, defined “acts of terrorism” as “criminal acts directed against a 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.” The scope of the definition of terrorism (in Article 2) extended beyond acts of violence against persons to include damage to public property. It made no reference to terrorism in war.
The 1979 Hostage-Taking Convention was the first overt attempt by the UN General Assembly to prescribe an offence with a terrorist motive that seeks to compel conduct by others through the unlawful action of detaining a person and using their safety or freedom as a bargaining chip. It described hostage-taking as a “manifestation” of international terrorism, albeit only in a preambular paragraph.
The material scope of the Convention is not limited to peacetime, even though during the drafting some States proposed the general exclusion of its application during armed conflict. Instead, the text does not govern acts of hostage-taking in armed conflict where States parties to the salient LOAC treaties are obligated to either prosecute or hand over for prosecution a hostage-taker. This pertains to international armed conflict (IAC), explicitly including, by the terms of Article 12, situations where a people are fighting in the exercise of their right of self-determination. Taking as hostage a civilian in occupied territory or on the territory of the parties to an IAC, where that civilian is a protected person, is a grave breach of 1949 Geneva Convention IV (GC IV).
The exclusionary clause in the 1979 Hostage-Taking Convention does not apply in non-international armed conflict (NIAC), and therefore incidents of hostage-taking in such conflicts would potentially fall within the Convention’s scope. This means the taking as hostage of members of State armed forces or non-State armed groups that are party to such a conflict also falls within the remit of this sectoral terrorism convention, unless the offence “is committed within a single State, the hostage and the alleged offender are nationals of that State and the alleged offender is found in the territory of that State.” Article 13 generally excludes such so-called “domestic” terrorism from the purview of the Convention.
The first UN treaty to contain the word “terrorist” in its title was the 1997 International Convention for the Suppression of Terrorist Bombings. This Convention, negotiated at the instigation of the United States following the bombing of its air force personnel at the Khobar Towers in Dhahran, Saudi Arabia, in 1996, requires the criminalization in domestic law of the act of,
… intentionally delivering, placing, discharging, or detonating an explosive or other lethal device in, into or against a place of public use, a State or government facility, a public transportation system or an infrastructure facility, with the intent to cause death or serious bodily injury or to cause extensive destruction of such a place, facility or system, where such destruction results in or is likely to result in major economic loss.
Apart from the intention to commit the proscribed act delineated in Article 2(1), the commission of the offence does not require a separate terrorist motivation.
The 1997 Terrorist Bombings Convention is also notable for its more general exclusion of situations of armed conflict from its material scope. Thus, Article 19(2) provides: “The activities of armed forces during an armed conflict, as those terms are understood under international humanitarian law, which are governed by that law, are not governed by this Convention.”
While at first view this is a comprehensive exclusion and a deference to LOAC, in fact, this is not the case. True, despite opposition from a small number of commentators (notably see here and here), the notion of “armed forces” does encompass both State armed forces and organized non-State armed groups, where they are party to any armed conflict. This is clear from the text of Article 19(2), which refers first to “armed forces” and then to the “military forces of a State.” But a civilian who decides to engage in prohibited conduct in bello but who is not a member of any armed forces would fall outside the exclusion. This would mean, for instance, that Ukrainian civilians who throw Molotov cocktails at Russian forces in the context of the current IAC on Ukrainian soil are engaging in international terrorism. The treaty accordingly dictates their prosecution by the relevant authority.
It is the 1999 Terrorism Financing Convention, however, that has the greatest resonance in armed conflict. This is because the definition of terrorism in Article 2(1)(b) explicitly applies its provisions to any,
… 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.
The 1999 Convention is by a distance the most widely ratified sectoral treaty on the repression of terrorism with 190 State parties. At the time of writing, only Burundi (a signatory), Chad, Eritrea, Iran, Somalia (also a signatory), the State of Palestine, and Tuvalu were not party. None of the sectoral treaties, though, institutes an international crime of terrorism.
Terrorism under the Law of Armed Conflict and International Criminal Law
Turning to LOAC, after the end of the Second World War, the four 1949 Geneva Conventions contained only a single mention of a prohibition on terrorism, but its inclusion was nonetheless significant. Article 33 of GC IV provides that “all measures of intimidation or of terrorism are prohibited.” Incorporated within Section I of the Fourth Convention, this rule pertains to civilians as “protected persons” in the territories of the parties to an IAC and to territories subject to belligerent foreign occupation. Article 3 of the Convention, which makes provision for NIACs, includes no corresponding stipulation.
The repression of “measures” of terrorism against persons in the power of the enemy is a rather awkward formulation, which other LOAC treaties have not repeated. Instead, the two 1977 Additional Protocols prohibit “acts” of terrorism, which is a more readily understandable concept. The brief discussion in the 1958 commentary of Jean Pictet on Article 33 of GC IV does little to clarify the scope of the rule, but the claimed rationale is narrow, based on the affirmation that “in resorting to intimidatory measures to terrorise the population, the belligerents hoped to prevent hostile acts.” The grave breaches provisions in GC IV do not explicitly include terrorism as a basis for the aut dedere, aut iudicare duty of criminal suppression. Although, as mentioned, hostage-taking does fall within the grave breaches provisions. These are set forth in Article 147 of GC IV on penal sanctions.
The 1977 Additional Protocols both explicitly prohibit terrorism. In the conduct of hostilities, the formulation in Article 51(2) of Additional Protocol I (AP I) and Article 13(2) of Additional Protocol II (AP II) is identical: “Acts or threats of violence the primary purpose of which is to spread terror among the civilian population are prohibited.” The International Committee of the Red Cross (ICRC) avers that State practice establishes this as a customary rule of international law applicable in both IAC and NIAC.
AP I does not address terrorism under the Geneva law branch of LOAC, presumably because a prohibition already exists under GC IV. Article 4(2)(d) of AP II, however, prohibits both “acts of terrorism,” which are distinct from a prohibition on the taking of hostages in subparagraph (c), as well as “threats” to commit terrorism under subparagraph (h). The scope of the rules is broad. It is not limited to unlawful acts against civilians but also extends to persons who have ceased to take part in hostilities.
This more extensive application to former fighters also features in the 1999 Terrorism Financing Convention. Inexplicably, however, the ICRC Customary International Humanitarian Law study did not find that the prohibition of acts of terrorism was a distinct customary rule (as it assuredly is). That said, an ICRC commentary on the customary rule against terror attacks does refer to a “wider prohibition” of measures or acts of terrorism. But in no case did the ICRC find that either terror attacks or acts of terrorism were war crimes under customary law.
Indeed, despite the significance and customary nature of the primary rule, neither AP I nor AP II criminalizes acts of terrorism or terror attacks. The grave breaches provisions of AP I added several war crimes, including direct attacks on civilians and attacks where it is known they will have a disproportionate impact on civilians. Article 85(5) further provides that grave breaches of the Geneva Conventions and AP I “shall be regarded as war crimes.” But none involves terror specifically. Nevertheless, when asked to draw up a list of war crimes for the putative jurisdiction of the International Criminal Tribunal for Rwanda (ICTR), the UN Secretariat decided this list should include “acts of terrorism” in Article 4(d) of the ICTR Statute, as well as threats to commit terrorism in Article 4(h).
There are of course no grave breaches provisions whatsoever in AP II. Even so, the ICTR Statute was adopted under UN Security Council Resolution 955 (1994) with both Geneva Law war crimes (acts of terrorism and threats to commit terrorism) included. The abstention in the Security Council vote by China was not related to the issue of terrorism under the Statute, while Rwanda’s vote against the resolution was not directly related to it, but part of broader opposition to inclusion of all international crimes other than genocide. Ultimately, however, the ICTR did not convict anyone under the terrorism provisions in its Statute.
It is necessary to distinguish the Rwanda tribunal jurisprudence from that under the Special Court for Sierra Leone (SCSL), where the authorities successfully prosecuted several accused for acts of terrorism. The SCSL Statute, adopted in 2002 as an annex to an agreement between the UN and the Government of Sierra Leone, similarly provided the hybrid court with material jurisdiction over the war crimes of acts of terrorism and threats to commit terrorism, under Article 3(d) and Article 3(h) respectively. The armed conflicts in Sierra Leone in the late 1980s and the 1990s had certainly seen widespread use of terror tactics by non-State armed groups, in particular the Revolutionary United Front (RUF) and the Armed Forces Revolutionary Council (AFRC). These included frequent amputations of the arms of civilian women and children, including during a January 1999 offensive against Freetown.
A number of AFRC commanders were put on trial before the SCSL, leading to the 2007 judgment in the so-called “AFRC case.” The indictment against the three defendants accused them of having ordered the carrying out of armed attacks primarily to terrorize the civilian population. The prosecution further alleged that as part of its campaign of terror and punishment, the AFRC/RUF routinely captured and abducted members of the civilian population; raped women and girls; and used many of the women and girls they abducted as sex slaves. It further alleged the AFRC/RUF used men and boys they had abducted as forced labour and gave combat training to abducted children who they forced to engage in active fighting. The AFRC/RUF physically mutilated men, women, and children, including amputating the hands or feet of many and carving “AFRC” and “RUF” on their bodies.
In delineating the crime of terror under the SCSL Statute, the Special Court borrowed from the jurisprudence of the International Criminal Tribunal for the former Yugoslavia (ICTY) (discussed below). However, the ICTY caselaw related not to acts of terrorism but to terror attacks against the civilian population during the conduct of hostilities. Thus, in paragraph 1440 of its judgment, the SCSL Trial Chamber mistakenly adopted an approach to the crime of terror that foresaw the mens rea element of the offence as requiring proof beyond reasonable doubt of the “primary intent of spreading terror among the civilian population.”
This was a flawed decision in legal terms. It was also an error with consequence, leading the Chamber to conclude that the “primary purpose” of the conscription and use of child soldiers by the AFRC during the conflict was not to spread terror among the civilian population, but rather was primarily military in nature and thus not an act of terrorism. This is to downplay the consequences of the recruitment of the children (and of course, a fortiori, their subsequent treatment) as well as the impact on their families: all were foreseeably, and, one might say inevitably, terrorized because of the unlawful, ongoing conduct.
With respect to the commission of abductions and forced labour, the Trial Chamber likewise concluded that their primary purpose was not to spread terror among the civilian population, “but rather was primarily utilitarian or military in nature.” The Trial Chamber further held that the primary purpose behind the AFRC’s sexual enslavement was for its troops “to take advantage of the spoils of war, by treating women as property and using them to satisfy their sexual desires and to fulfil other conjugal needs.” I argue in my book, International Counterterrorism Law, that this egregious failure to comprehend the impact on the victims in assessing criminality resulted in a failure of criminal justice. That is so, notwithstanding the fact that the Trial Chamber did find that the amputations of the limbs of many civilians were indeed acts of terrorism.
In the case of the ICTY, the Statute did not expressly include any crime relating to terror or terrorism (although the Statute did include the taking of civilians as hostages in IAC). Nonetheless, prosecutors pursued the conviction of several defendants of terror attacks in the conduct of hostilities. With respect to the siege of Sarajevo, in 2003, an ICTY Trial Chamber convicted General Stanislav Galić, the commander of the Bosnian Serb army around the capital of Bosnia and Herzegovina, of terror attacks for having visited “a protracted campaign of shelling and sniping upon civilian areas of Sarajevo and upon the civilian population thereby inflicting terror and mental suffering upon its civilian population.” The manner in which the Court did so, however, defied fundamental principles of criminal justice, holding, bizarrely, that the treaty prohibition had been implicitly criminalized by AP I (see, inter alia, paras. 69, 97, 98, and 113 of the 2003 judgment).
In his thoughtful 2006 volume, Defining Terrorism in International Law, Ben Saul charitably described this approach as “not entirely persuasive” (p. 305). It was clear, however, that this breach of nullum crimen sine lege could not survive on appeal. Indeed, the Appeals Chamber took a markedly different tack, declaring itself satisfied (at para. 86 of its judgment) that “a breach of the prohibition of terror against the civilian population gave rise to individual criminal responsibility pursuant to customary international law at the time of the commission of the offences for which Galić was convicted.”
In both the Trial Chamber and the Appeals Chamber, conviction was by majority decision. In his persuasive dissenting opinion in the Trial Chamber, Judge Rafael Nieto-Navia argued that the crime of terror attacks did not fall within the jurisdiction of the ICTY on the basis that the evidence presented by the prosecution and the majority of the Trial Chamber did not establish that the offence of inflicting terror on a civilian population attracted individual criminal responsibility under international law.
In the Appeals Chamber, in his corresponding dissenting opinion, Judge Wolfgang Schomburg declared there was no basis to find that “terrorization against a civilian population” was unlawful “beyond any doubt under customary international criminal law at the time relevant to the Indictment.” He agreed there could be “no doubt” that the prohibition per se was part of customary law but questioned the State practice on which the majority of the Appeals Chamber relied, observing, inter alia, that none of the permanent members of the UN Security Council “or any other prominent State” had proscribed terrorization against a civilian population as a war crime. He also considered it relevant that the Rome Statute did not have jurisdiction over such a war crime (see paras. 2, 7, 8–12, 18, and 20 of the dissent).
Despite these objections, the ICTY would consistently hold that terror attacks were indeed war crimes under customary law. In June 2021, in adjudicating the appeal against conviction of Ratko Mladic, the commander of the Bosnian Serb army in Bosnia and Herzegovina, the Residual Mechanism of the ICTY that followed the Tribunal’s closure once again addressed the issue of whether terrorizing civilians in the conduct of hostilities was indeed a customary law war crime. Mr. Mladic had asserted the prohibition of spreading terror among the civilian population did not extend to its criminalization under customary international law, at least when the Siege of Sarajevo was ongoing, “due to insufficient evidence of settled, extensive, or uniform state practice” (para. 280). The Residual Mechanism, however, rejected his assertion.
Whether other international criminal tribunals will follow the approach of the ICTY (and whether, for instance, States will amend the Rome Statute to provide for material jurisdiction over such a war crime) presently remains unclear.
Prosecuting the Planning of and Complicity in the 9/11 Attacks
This segues to consideration of the specific charges of terrorism as war crimes laid against those who planned or assisted the 9/11 attacks against the United States. On 31 May 2011, following the enactment of the National Defense Authorization Act, the Criminal Investigation Task Force charged Khalid Sheikh Mohammed and four others for their involvement in the 9/11 attacks. These charges included, inter alia, terrorism, conspiracy to commit terrorism, and hijacking an aircraft, all as war crimes.
The charges against the 9/11 planners, especially Mr. Mohammed, are, by their nature, predicated on the existence of a prior armed conflict. This is a significant obstacle to securing a conviction. But the definition of the war crime of terrorism is also problematic. The Military Commissions Act 2009 (at §950t(24)) defined the crime as the intentional killing or infliction of “great bodily harm on one or more protected persons,” or intentionally engaging in an “act that evinces a wanton disregard for human life, in a manner calculated to influence or affect the conduct of government or civilian population by intimidation or coercion, or to retaliate against government conduct … .” The term “protected person” is in turn defined to mean “any person entitled to protection under one or more of the Geneva Conventions, including civilians not taking an active part in hostilities, military personnel placed out of combat by sickness, wounds, or detention, and military medical or religious personnel.”
The U.S. definition strays markedly from the definition of both “acts of terrorism” and terror attacks under LOAC and their corresponding criminalization, to the extent they are indeed war crimes, under customary international criminal law. In neither case is the influencing of government conduct understood as a terrorist motivation in LOAC; the language derives from the UN sectoral treaties on terrorism, which does not involve international crimes. That the United States is not a party to the 1977 Additional Protocols, which address the conduct of hostilities, is notable, even if it does not preclude the customary law nature of the crimes.
With respect to the passengers on the four planes, they would not formally be “protected persons” under the Geneva Conventions insofar as any conflict with al-Qaeda would be of a non-international character. Moreover, at the least with respect to the attack on the Pentagon on 9/11, this concerned the targeting of a military objective and U.S. military personnel, who are not “protected persons” for the purposes of the Geneva Conventions. If the cases ever come to court, it will be enlightening to see how the Military Commission treats the U.S. definition of the crimes.
Concluding Thoughts
A review of efforts to regulate terrorism through international law reveals a variety of approaches, including by widely ratified treaties and by international criminal law tribunals. The questions whether terrorism is cognizable as an offence during armed conflict and whether it constitutes an offence in customary international law that may be criminally enforced against individuals remain unsettled even today. That terrorism persists as a tactic in so many ongoing armed conflicts with such devastating effects on humanity presents a compelling cause for resolution of these questions by States.
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Stuart Casey-Maslen is Visiting Professor at the Faculty of Law of University of Johannesburg.
The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.
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