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On December 22, 2001, Richard Reid, a Muslim from the United Kingdom, boarded an American Airlines flight with explosives in his shoes. His goal was to blow up the plane. As the flight departed France, en route to Miami, Reid tried to detonate his shoe bomb, but the smell and smoke attracted attention and Reid was subdued by the passengers.

On that day, an act of terror was thwarted. But why did Reid do it? In this article, we will explore the history and types of terrorism and the development of counterterrorism.

Terrorism Definition

Terrorism is differentiated from conventional warfare because terrorists usually specifically target unarmed civilians rather than military forces or installations.

Terrorism is the use or threat of unpredictable acts of violence, usually against non-military/civilian populations, to achieve a political or religious goal.

Terrorists are not trying to win wars through strength in arms or military doctrine. The main purpose of terrorism is to instill fear. Terrorist organizations want ordinary citizens to fear that a terrorist attack could happen anytime, anywhere—and the only way to stop that is to comply with the terrorists' demands.

The word "terrorism" emerged during the 18th century to describe the French Republic's use of terror and violence to coerce French citizens into civil compliance. Nowadays, the term almost exclusively refers to non-state individuals or organizations.

Terrorism History

Terrorists do sometimes attack military targets, especially if the terrorists want a foreign occupational military to leave. The principle is the same: terrorists want the fear and mistrust felt by foreign military troops to be so overwhelming that their government is forced to reevaluate the cost and value of the war/occupation.

If you are acquainted with the US military's operations in Afghanistan, that all may sound familiar. The initial stages of conventional warfare eventually gave way to semi-frequent terrorist attacks on US troops and bases. This type of terrorism stretches back several thousand years, and is, in fact, the origin of terrorism as a political tool.

Ancient and Medieval Terrorism

Terrorism was conceived by the Sicarii Zealots, Jewish extremists who assassinated and terrorized Roman military forces, Roman civilians, and Jewish Roman sympathizers at public gatherings in the 1st century AD. Their goal was to force the Romans to withdraw from Palestine.

Similarly, the Order of the Assassins (also called the Hashashins or just Assassins), operated throughout southwest Asia from 1090 AD to 1275 AD. Practicing a radical form of Shia Islam, the Assassins killed Sunni Muslim or Christian leaders they perceived as threats to their financial independence or religious beliefs—acts of terror meant to convince others to leave the Assassins alone.

The earliest forms of terrorism, as you can see, are difficult to distinguish from assassinations and/or guerilla warfare. Assassination is the deliberate targeting and killing of an individual as an act of political, economic, or religious subterfuge. Guerilla warfare is unconventional warfare in which non-traditional military tactics, like ambushing or hit-and-run, are used to try to defeat a more powerful force. Oftentimes, terrorism, assassinations, and guerilla warfare intermingle.

Terrorism in the 20th Century

Terrorism became a common tactic of the original Ku Klux Klan (KKK) in the United States from 1865 to 1871. The KKK used terror and violence to prevent African Americans from voting and to influence local communities to uphold segregation and racial discrimination. The second iteration of the KKK, active from 1915 to the 1940s, largely used the same tactics but expanded their campaigns of terror to include virtually anyone who was not white/Anglo-Saxon and Protestant.

After the 1940s, terrorist activity became more standardized:

  • Well-defined paramilitary or terrorist groups organized around a specific political or religious cause

  • Planned but indiscriminate acts of violence at seemingly random times and locations

  • Terrorists usually indistinguishable from the local population by using civilian attire rather than a uniform

  • The use of munitions to kill large numbers of civilians at once

  • Ideology and demands widely conveyed via media (newspapers, radio, television, and later, the internet)

During the 20th century, an era fraught with political turmoil, most acts of terror were motivated by separatism, the desire for political independence, and/or ethnic sovereignty. The Real Irish Republican Army (RIRA), a group seeking the reunification of Ireland, carried out terrorist attacks in UK-controlled Northern Ireland. Similarly, Euskadi Ta Askatasuna (ETA), a radical Marxist organization, attacked civilians throughout Spain in support of Basque independence. Hezbollah, based in Lebanon, has committed acts of terror in Israel in support of Palestinian sovereignty.

Terrorism, Terrorism History, ETA, StudySmarterFig. 1 - Members of ETA hold a rally in 2006

Several of these conflicts are still ongoing and many of the terrorist organizations that emerged during the 20th century are still active.

War on Terrorism

The terrorist attacks on September 11, 2001 largely shifted the world's focus away from political separatist-based terrorism toward religious extremist-based terrorism (although the two often coincide).

On the morning of September 11th, terrorists from al-Qaeda took control of four in-flight aircraft and diverted them toward major US infrastructure. Two planes were flown into the Twin Towers of the World Trade Center in New York City, which subsequently collapsed. A third plane was flown into the Pentagon in Arlington, Virginia. The fourth plane, headed toward the Capitol, was commandeered by civilian passengers and crashed into a field in Pennsylvania.

Terrorism, Terrorism History, War on Terrorism, 9/11, 9/11 Map, StudySmarterFig. 2 - Four flights were hijacked by al-Qaeda operatives on September 11, 2001. The numbers show the order in which the events took place

The September 11th attacks (commonly referred to as simply 9/11) remain the single deadliest act of terror in history. Around 3,000 people were killed.

Al-Qaeda is an Islamic extremist terrorist organization. Like most other Islamic extremist organizations, al-Qaeda seeks to unite the world (and especially southwest Asia and North Africa) under a single Islamic caliphate with strict adherence to traditional Islamic religious law (sharia) and to punish those who resist. Al-Qaeda perceives the globalization of Western, and especially American, culture as a threat to their aims.

Al-Qaeda has long harbored enmity toward the United States. The precursors to 9/11 include al-Qaeda's 1993 bombing of the World Trade Center, the 1998 attack on US embassies in East Africa, and the 2000 suicide bombing of the USS COLE, a US Navy destroyer.

to confront Islamic extremism (and other forms of terrorism) throughout the world. The War in Afghanistan (2001-2021) was part of this War on Terror. The US and its allies invaded Afghanistan to confront al-Qaeda and the Taliban, a group that had allowed al-Qaeda to use Afghanistan as a base of operations.

Terrorism, Terrorism History, Global War on Terrorism, US Army Patrol, StudySmarterFig. 3 - US Army soldiers patrol a highway in Afghanistan

The Global War on Terror is ongoing and has had only mixed success. Western forces and their global partners have managed to prevent something on the scale of 9/11 from happening a second time. But they have neither eliminated key terrorist groups nor have they stopped terrorism altogether. Strategic blunders and indiscriminate military strikes in southwest Asia and elsewhere have, in some cases, exacerbated the sour relations between the West and the Muslim world, providing even more motivation for Islamic extremists.

Al-Qaeda itself remains active, and since 2001, new Islamic extremist groups like Boko Haram, al-Shabaab, and the Islamic State in Syria and the Levant (ISIS or ISIL) have committed acts of terror throughout the world in the name of the united Islamic caliphate and the wholesale rejection of Western globalization and perceived immorality.

The Future of Terrorism

The rise of social media has made it easier for people to find and identify with extremist ideologies. Internet-based radicalization has increased the prevalence of lone offenders ("lone wolves"), individuals who may agree with a terrorist organization but are not explicitly associated with them.

Dzhokhar Tsarnaev and Tamerlan Tsarnaev, who committed the 2013 Boston Marathon Bombing, were motivated by Islamic extremism but were not affiliated with any specific terrorist organizations.

In the past several years, there has been an increase in extreme right-wing terrorist attacks, which are more likely to be committed by lone offenders who have been radicalized through internet activity. In 2019, Patrick Wood Crusius killed 23 people in El Paso, Texas to protest Hispanic immigration to the US.

What is the difference between a terrorist attack and a mass shooting? Many terrorist attacks are mass shootings, but not all mass shootings are terrorist attacks. Remember, the purpose of a terrorist attack is to evoke fear in hopes of soliciting a political response. Many mass shootings or rampages are conducted to take "revenge" on a society that is perceived as specifically unjust toward the perpetrator. The Columbine High School massacre in 1999, for example, had no obvious political or religious objective.

Crusius' attack was also tangentially related to the environment. He believed the same corporations and politicians encouraging Hispanic immigration were also destroying the natural world. Eco-terrorism is a form of terrorism that promotes the use of terror to encourage environmental protection. As environmental degradation continues, it is possible that eco-terrorism will become more common.

Types of Terrorism

From a geographic perspective, there are two main types of terrorism: domestic terrorism and international terrorism.

Domestic Terrorism

Domestic terrorism is terrorism perpetrated by a citizen or citizens of a country against fellow citizens.

The 1995 Tokyo Subway sarin gas attack was perpetrated by Aum Shinrikyo, a Japanese cult and terrorist organization, against other Japanese citizens.

International Terrorism

Conversely, international terrorism is terrorism perpetrated against the citizens of a country by a foreign or multinational terrorist group.

In 2013, al-Shabaab terrorists attacked the Westgate shopping mall in Kenya. Al-Shabaab is based in Somalia; the terrorists included a Sudanese national, a Somali, and a Kenyan Arab.

Terrorism as a Devolutionary Factor

Terrorism can cause devolution, the process through which a region is given autonomy or independence by a central government. Indeed, as we've mentioned, devolution is the leading motive of terrorist organizations like ETA and RIRA.

However, it is important to note that terrorism is usually unsuccessful. Because terrorism is so extreme, it may have the opposite of its intended effect: political dialogue may cease entirely, and governments may become even more intrusive in potentially-devolving regions in attempts to restore stability and control.

Other devolutionary factors include irredentism, economic and social instability, isolation due to physical geography, and non-terrorist-related ethnic separatism.


Terrorists' biggest weapon is not violence, but fear. You have only about a .01% chance of being killed in a terrorist attack: the fear these attacks create is not proportional to their actual likelihood.

That is not to dismiss the very real lives that have been lost to terrorism. A .01% chance is still too high. Since the late 19th century, governments have been developing methods to stop terrorist attacks, a practice known as counterterrorism. Counterterrorism relies on a wide array of techniques—including intelligence gathering, apprehension and arrests, humanitarian aid, and propaganda—to deter or prevent terrorist attacks from occurring. Military and police forces may be trained to fight terrorists in the event that an attack does occur.

Terrorism, Counter Terrorism, Serbian Police, StudySmarterFig. 4 - Members of the Serbian police train for counterterrorist urban warfare

Many counterterrorist methods are controversial. Advanced interrogation techniques, usually indistinguishable from torture, can be used to solicit intelligence from captured terrorists, but raise concerns over human rights. Surveillance and monitoring of potential terrorists may lead to the loss of privacy for people who have nothing to do with terrorism. Racial and religious profiling can lead to discrimination and racism.

Terrorism - Key takeaways

  • Terrorism is the use or threat of unpredictable acts of violence, usually against non-military/civilian populations, to achieve a political or religious goal.
  • Terrorism originated with the Sicarii Zealots in 1st century Palestine. Terrorism is a devolutionary factor but is not usually successful due to its extreme nature.
  • The Global War on Terror has been ongoing since 2001 and revolves around allied efforts to confront terrorism around the world, especially Islamic extremism.
  • The two major types of terrorism are domestic terrorism and international terrorism.
  • Counterterrorism emerged as a way to prevent terrorism and/or fight terrorists.


  1. Fig. 1, Arma Tiro Pum ETA (https://commons.wikimedia.org/wiki/File:ArmaTiroPumEtaMilikiliklik26_8.jpg), by un usuario de Indymedia Barcelona, Licensed by CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/deed.en)
  2. Fig. 4, SAJ vježba Dan Policije (https://commons.wikimedia.org/wiki/File:SAJ_vje%C5%BEba_Dan_Policije_2019.jpg), by the Ministry of Defence of Republic of Serbia, Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)

Frequently Asked Questions about Terrorism

Terrorism is the use or threat of unpredictable acts of violence, usually against non-military/civilian populations, to achieve a political or religious goal. 

Terrorists are trying to instill fear, rather than win a war through strength in arms or military doctrine. Terrorists also usually intentionally target civilians. 

Terrorist attacks can cause very real deaths, which can create widespread fear. Societies develop counterterrorism practices to prevent future terrorist attacks. Counterterrorism may result in loss of privacy and sometimes lead to human rights violations. 

The Global War on Terror (2001-present) is the term for the collective efforts of the US government and its global partners to confront terrorism throughout the world, especially Islamic extremist-based terrorism. 

Counterterrorism includes the methods used to prevent or stop terrorist tasks. Counterterrorism includes intelligence gathering, apprehension and arrests, humanitarian aid, propaganda, conventional military attacks, and advanced interrogation techniques. 

Final Terrorism Quiz


Which of the following are characteristic of terrorist attacks? Select ALL that apply. 

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Designed to instill fear

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Which of the following terrorist groups are explicitly motivated by political devolution? Select ALL that apply. 

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Show question


True or False: The Sicarii Zealots operated in and around Poland during World War II. 

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False! The Sicarii Zealots operated in Palestine in the 1st Century AD. 

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Which terrorist organization was responsible for 9/11?

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Show question


The 9/11 attacks directly led to which of the following conflicts? Select all that apply. 

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The Global War on Terror

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At the time of the 9/11 attacks, al-Qaeda was believed to be primarily operating out of which country?

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Show question


In the context of terrorism, what is a lone offender/lone wolf? 

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A lone offender is a terrorist who agrees with the ideology of a terrorist organization but is not explicitly affiliated with them. 

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Aum Shinrikyo's 1995 Tokyo Subway sarin gas attack was an example of what type of terrorism? 

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Domestic terrorism

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Why are advanced interrogation techniques controversial?

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They can be interpreted as torture and a violation of human rights

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What is international terrorism?

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International terrorism is terrorism perpetrated against the citizens of a country by a foreign or multinational terrorist group. 

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