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The origins of terrorism have been described in the following manner:
Few words are plagued by so much indeterminacy, subjectivity, and political disagreement as ‘terror’, ‘terrorize’, ‘terrorism’, and ‘terrorist’. The ordinary linguistic meanings of these variant terms are instantly evocative and highly emotive, referring at a literal level to intense fear, fright, or dread. By itself, a literal meaning is not particularly instructive in distilling a legal concept of terrorism, since ‘every form of violence is potentially terror-inspiring to its victim’, from mugging to warfare. This deceptively simple, literal meaning is overlaid with centuries of political connotations in specific historical circumstances. While the word ‘terror’ stems from Latin, entering French and English in the fourteenth century, the notions of ‘terrorism’ and ‘terrorist’ entered political discourse in the late eighteenth century, referring to the system of intimidation and repression implemented by the Jacobins (the ‘Red Terror’ or ‘Reign of Terror’) in the French Revolution. The idea of terrorism as an instrument of State control persisted until the end of the Second World War. Bismarck ‘terrorized’ Prussia by using the army as a means of social control; Nazi Germany imposed a reign of terror across Europe; German and Allied air forces resorted to ‘terror bombing’ in the Second World War; and Stalin ruled Russia by terror.
Terrorism appeared on the international agenda in 1934 for the first time, when the League of Nations put forward a plan to draft a convention for the prevention and punishment of terrorism. In 1937, an agreement was reached on the Convention for the Prevention and Punishment of Terrorism. The Convention, however, never received sufficient ratifications to enter into force.
In 1972, the General Assembly set up a committee on terrorism which operated until 1979, when operations came to an end due to lack of agreement on the definition of terrorism.
Although there is no international convention that prohibits terrorism as such, numerous instruments, outlawing certain aspects of terrorism, have been adopted in order to prevent and suppress terrorist acts. To this extent, numerous international treaties were agreed upon which address specific areas of terrorist activities. The earliest of these conventions were the 1970 Convention for the Suppression of Unlawful Seizure of Aircraft (the Hague Convention) and the 1971 Convention for the Suppression of Unlawful Acts against the Safety of Civil Aviation (the Montreal Convention). These treaties address terrorist offences committed during air travel (for instance, hijacking).
Two other relevant conventions are the 1997 International Convention for the Suppression of Terrorist Bombings and the 1999 International Convention for the Suppression of the Financing of Terrorism.
These multilateral Conventions strongly rely on inter-State cooperation and impose the obligation of extradite or prosecute (aut dedere aut judicare), which obliges States to pursue a national prosecution of terrorism either through their own courts or through the courts of other States willing to prosecute.
Acts of terrorism can be currently prosecuted by the international criminal tribunals for the former Yugoslavia (ICTY) and Rwanda (ICTR) only if they amount to crimes against humanity and/or war crimes (see Arts. 2, 3, and 5 of the ICTY Statute and Arts. 3 and 4 of the ICTR Statute). The only exception is the Special Tribunal for Lebanon (STL), which has jurisdiction over terrorist acts (see Art. 2(a) of the STL Statute). However, the Special Tribunal acts under Lebanese domestic law and not international law. The International Criminal Court (ICC) does not have jurisdiction over terrorism as a distinct crime. Terrorism is also excluded from the list of war crimes provided in Article 8 of the Rome Statute of the International Criminal Court. The only manner in which the International Criminal Court could exercise jurisdiction over acts of terrorism is if terrorist acts would amount to another crime over which the Court has jurisdiction (for instance, crimes against humanity under Article 7 of the Rome Statute).
Providing a definition of terrorism presents a considerable difficulty and legal scholars have differing views on the matter. As of now, no global definition of terrorism has been agreed upon. Furthermore, the problem is intensified by the unique nature of terrorism, since “one man’s terrorist is the other man’s freedom fighter”. In a recent article, it was considered that:
At first sight it may seem obvious as to what should be considered as an act of terrorism. As former British Ambassador Sir Jeremy Greenstock said, ‘what looks, smells and kills like terrorism, is terrorism’. The ordinary textual meaning of ‘terrorism’ refers to extreme fear. A literal meaning is not, however, particularly helpful in legally defining terrorism, since many forms of violence, from mugging to rape to piracy, can cause terror, yet such crimes are not generally thought of as ‘terrorism’.
Disagreement about terrorism is not simply focused upon linguistics, but also basic moral, political and ideological questions about the legitimacy of using violence. Reaching global legal agreement on defining terrorism presupposes agreement about who is entitled to use violence, against whom, and for what purposes. In a world of diverse political, moral and cultural systems, this is no easy task, and explains why disagreement persists.
In the aftermath of the 9/11 attacks, the United Nations Security Council adopted Resolution 1373 (2001), calling the events in New York, Washington D.C. and Pennsylvania “terrorist attacks”and constituting a threat to international peace and security.
In its Resolution 1566 (2004), the Security Council drew on the international conventions and protocols relating to terrorism, and on this basis, referred to 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.
This definition was endorsed by the Secretary-General in his report of the High-level Panel on Threats, Challenges and Change in 2004.In para. 164, subparagraph (d) it provides that terrorism is:
any action, in addition to actions already specified by the existing conventions on aspects of terrorism, the Geneva Conventions and Security Council resolution 1566 (2004), that is intended to cause death or serious bodily harm to civilians or non-combatants, when the purpose of such an 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.
Perhaps the most debated definition of terrorism, as an international crime, has been brought forward in the landmark decision of the Appeals Chamber of the Special Tribunal for Lebanon (STL). In paragraph 85 of that decision, the Appeals Chamber considers that:
… the international crime of terrorism … requires the following three key 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.
On the international level, terrorist acts have been prosecuted as war crimes and/or crimes against humanity.
The first case before an international tribunal involving terrorism was the case against General Galić. The International Criminal Tribunal for the former Yugoslavia (ICTY) convicted General Galić of terrorism as a war crime and crimes against humanity for the acts of violence the primary purpose of which is to spread terror among the civilian population. General Galić was responsible for the campaign of shelling and sniping of civilians in Sarajevo (Bosnia and Herzegovina) between 1992 and 1994. The ICTY considered this campaign as an act of terrorizing the civilian population (see para. 178 et seq. of the Judgement of the ICTY Trial Chamber).
On a national level, some of the well-known terrorism related cases include the prosecution of the self-described mastermind of the 9/11 attacks, Khalid Shaikh Mohammed or Richard Reid, the British “shoe bomber” who attempted to blow up an American Airlines flight in December 2001.
For more information on the legal aspects in countering terrorism, see here.
Convention on Offences and Certain Other Acts Committed on Board Aircraft, signed at Tokyo on 14 September 1963. (Deposited with the Secretary-General of the International Civil Aviation Organization)
Convention for the Suppression of Unlawful Seizure of Aircraft, signed at the Hague on 16 December 1970. (Deposited with the Governments of the Russian Federation, the United Kingdom and the United States of America)
Convention for the Suppression of Unlawful Acts against the Safety of Civil Aviation, signed at Montreal on 23 September 1971. (Deposited with the Governments of the Russian Federation, the United Kingdom and the United States of America)
Protocol on the Suppression of Unlawful Acts of Violence at Airports Serving International Civil Aviation, supplementary to the Convention for the Suppression of Unlawful Acts against the Safety of Civil Aviation, signed at Montreal on 24 February 1988.
Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation, done at Rome on 10 March 1988. (Deposited with the Secretary-General of the International Maritime Organization)
Protocol for the Suppression of Unlawful Acts against the Safety of Fixed Platforms Located on the Continental Shelf, done at Rome on 10 March 1988. (Deposited with the Secretary-General of the International Maritime Organization)
Convention on the Marking of Plastic Explosives for the Purpose of Detection, signed at Montreal on 1 March 1991. (Deposited with the Secretary-General of the International Civil Aviation Organization)
 Ben Saul, Defining Terrorism in International Law, Oxford University Press 2008, p. 2.
 For further sources see, e.g., B. Saul, Defining Terrorism in International Law (Oxford University Press 2008); A. Cassese, International Criminal Law (2nd ed., Oxford University Press, Oxford 2008) 162-187; R. Cryer et al., International Criminal Law and Procedure (2nd ed., Cambridge University Press, Cambridge 2010) 336-352.