Counter-terrorism (also spelled counterterrorism), also known as antiterrorism, incorporates the practice, military tactics, techniques, and strategy that government, military, law enforcement, business, and intelligence agencies use to combat or prevent terrorism. Counter-terrorism strategies include attempts to counter financing of terrorism.

If terrorism is part of a broader insurgency, counter-terrorism may employ counter-insurgency measures. The United States Armed Forces use the term foreign internal defense for programs that support other countries in attempts to suppress insurgency, lawlessness, or subversion or to reduce the conditions under which these threats to security may develop.^[1]

In response to the escalating terror campaign in Britain carried out by the militant Irish Fenians in the 1880s, the Home Secretary, Sir William Harcourt, established the first counter-terrorism unit ever. The Special Irish Branch was initially formed as a section of the Criminal Investigation Department of the London Metropolitan Police in 1883, to combat Irish republican terrorism through infiltration and subversion.

Harcourt envisioned a permanent unit dedicated to the prevention of politically motivated violence through the use of modern techniques such as undercover infiltration. This pioneering branch was the first to be trained in counter-terrorism techniques.^[2]

Its name was changed to Special Branch as it had its remit gradually expanded^[3] to incorporate a general role in counterterrorism, combating foreign subversion and infiltrating organized crime. Law enforcement agencies, in Britain and elsewhere, established similar units.^[4]

Counterterrorism forces expanded with the perceived growing threat of terrorism in the late 20th century. Specifically, after the September 11 attacks, Western governments made counter-terrorism efforts a priority, including more foreign cooperation, shifting tactics involving red teams^[5] and preventive measures.^[6] Although sensational attacks in the developed world receive a great deal of media attention^[7], most terrorism occurs in less developed countries.^[8] Government responses to terrorism in some cases generate substantial unintended consequences.^[9]

Most counter-terrorism strategies involve an increase in standard police and domestic intelligence. The central activities are traditional: interception of communications, and the tracing of persons. New technology has, however, expanded the range of military and law enforcement operations.

Domestic intelligence is often directed at specific groups, defined on the basis of origin or religion, which is a source of political controversy. Mass surveillance of an entire population raises objections on civil liberties grounds. Homegrown terrorists, especially lone wolves are often harder to detect because of their citizenship or legal status and ability to stay under the radar.

To select the effective action when terrorism appears to be more of an isolated event, the appropriate government organizations need to understand the source, motivation, methods of preparation, and tactics of terrorist groups. Good intelligence is at the heart of such preparation, as well as political and social understanding of any grievances that might be solved. Ideally, one gets information from inside the group, a very difficult challenge for HUMINT because operational terrorist cells are often small, with all members known to one another, perhaps even related.^[10]

Counterintelligence is a great challenge with the security of cell-based systems, since the ideal, but nearly impossible, goal is to obtain a clandestine source within the cell. Financial tracking can play a role, as can communications intercept, but both of these approaches need to be balanced against legitimate expectations of privacy.

In response to the growing legislation.

Since 1978 the UK's terrorism laws have been regularly reviewed by a security-cleared Independent Reviewer of Terrorism Legislation, whose often influential reports are submitted to Parliament and published in full.

One of the primary difficulties of implementing effective counter-terrorist measures is the waning of civil liberties and individual privacy that such measures often entail, both for citizens of, and for those detained by states attempting to combat terror.^[14] At times, measures designed to tighten security have been seen as abuses of power or even violations of human rights.^[15]

Examples of these problems can include prolonged, incommunicado detention without judicial review or long periods of 'preventive detention';^[16] risk of subjecting to torture during the transfer, return and extradition of people between or within countries; and the adoption of security measures that restrain the rights or freedoms of citizens and breach principles of non-discrimination.^[17] Examples include:

Many would argue that such violations could exacerbate rather than counter the terrorist threat.^[17] Human rights advocates argue for the crucial role of human rights protection as an intrinsic part to fight against terrorism.^[18][19] This suggests, as proponents of human security have long argued, that respecting human rights may indeed help us to incur security. Amnesty International included a section on confronting terrorism in the recommendations in the Madrid Agenda arising from the Madrid Summit on Democracy and Terrorism (Madrid March 8–11, 2005):

While international efforts to combat terrorism have focused on the need to enhance cooperation between states, proponents of human rights (as well as human security) have suggested that more effort needs to be given to the effective inclusion of human rights protection as a crucial element in that cooperation. They argue that international human rights obligations do not stop at borders and a failure to respect human rights in one state may undermine its effectiveness in the international effort to cooperate to combat terrorism.^[17]

Some countries see preemptive attacks as a legitimate strategy. This includes capturing, killing, or disabling suspected terrorists before they can mount an attack. Israel, the United Kingdom, the United States, and Russia have taken this approach, while Western European states generally do not.

Another major method of preemptive neutralization is interrogation of known or suspected terrorists to obtain information about specific plots, targets, the identity of other terrorists, whether or not the interrogation subjects himself is guilty of terrorist involvement. Sometimes more extreme methods are used to increase suggestibility, such as sleep deprivation or drugs. Such methods may lead captives to offer false information in an attempt to stop the treatment, or due to the confusion brought on by it. These methods are not tolerated by European powers. In 1978 the European Court of Human Rights ruled in the Ireland v. United Kingdom case that such methods amounted to a practice of inhuman and degrading treatment, and that such practices were in breach of the European Convention on Human Rights Article 3 (art. 3).

The human security paradigm outlines a non-military approach which aims to address the enduring underlying inequalities which fuel terrorist activity. Causal factors need to be delineated and measures implemented which allow equal access to resources and sustainability for all people. Such activities empower citizens providing 'freedom from fear' and 'freedom from want'.

This can take many forms including the provision of clean drinking water, education, vaccination programs, provision of food and shelter and protection from violence, military or otherwise. Successful human security campaigns have been characterized by the participation of a diverse group of actors including governments, NGOs, and citizens.

Foreign internal defense programs provide outside expert assistance to a threatened government. FID can involve both non-military and military aspects of counter-terrorism.

A 2017 study found that "governance and civil society aid is effective in dampening domestic terrorism, but this effect is only present if the recipient country is not experiencing a civil conflict."^[20]

Terrorism has often been used to justify military intervention in countries like Pakistan where terrorists are said to be based. That was the main stated justification for the U.S. invasion of Afghanistan. It was also a stated justification for the second Russian invasion of Chechnya.

Military intervention has not always been successful in stopping or preventing future terrorism, like during the Malayan Emergency, the Mau Mau uprising, and most of the campaigns against the IRA during the Irish Civil War, the S-Plan, the Border Campaign (IRA) and the Troubles in Northern Ireland. Although military action can disrupt a terrorist group's operations temporarily, it sometimes doesn't end the threat completely.^[21]

Thus repression by the military in itself (particularly if it is not accompanied by other measures) usually leads to short term victories, but tend to be unsuccessful in the long run (e.g. the French's doctrine described in Roger Trinquier's book Modern War^[22] used in Indochina and Algeria). However, new methods (see the new Counterinsurgency Field Manual^[23]) such as those taken in Iraq have yet to be seen as beneficial or ineffectual.

Police, fire, and emergency medical response organizations have obvious roles. Local firefighters and emergency medical personnel (often called "first responders") have plans for mitigating the effects of terrorist attacks, although police may deal with threats of such attacks.

Whatever the target of terrorists, there are multiple ways of hardening the targets to prevent the terrorists from hitting their mark, or reducing the damage of attacks. One method is to place Hostile vehicle mitigation to enforce protective standoff distance outside tall or politically sensitive buildings to prevent car and truck bombing. Another way to reduce the impact of attacks is to design buildings for rapid evacuation.^[24]

Aircraft cockpits are kept locked during flights, and have reinforced doors, which only the pilots in the cabin are capable of opening. UK railway stations removed their rubbish bins in response to the Provisional IRA threat, as convenient locations for depositing bombs.

Scottish stations removed theirs after the 7 July 2005 London Bombings as a precautionary measure. The Massachusetts Bay Transportation Authority purchased bomb-resistant barriers after the September 11 terrorist attacks.

As Israel is suffering from constant shelling of its cities, towns and settlements by artillery rockets from the Gaza Strip (mainly by Hamas, but also by other Palestinian factions) and Lebanon (mainly by Hezbollah), Israel developed several defensive measures against artillery, rockets and missiles. These include building a bomb shelter in every building and school, but also deploying active protection systems like the Arrow ABM, Iron Dome and David's Sling batteries which intercept the incoming threat in the air. Iron Dome has successfully intercepted hundreds of Qassam rockets and Grad rockets fired by Palestinians from the Gaza Strip.

A more sophisticated target-hardening approach must consider industrial and other critical industrial infrastructure that could be attacked. Terrorists need not import chemical weapons if they can cause a major industrial accident such as the Bhopal disaster or the Halifax Explosion. Industrial chemicals in manufacturing, shipping, and storage need greater protection, and some efforts are in progress.^[25] To put this risk into perspective, the first major lethal chemical attack in WWI used 160 tons of chlorine. Industrial shipments of chlorine, widely used in water purification and the chemical industry, travel in 90 or 55 ton tank cars.

To give one more example, the North American electrical grid has already demonstrated, in the Northeast Blackout of 2003, its vulnerability to natural disasters coupled with inadequate, possibly insecure, SCADA (supervisory control and data acquisition) networks. Part of the vulnerability is due to deregulation leading to much more interconnection in a grid designed for only occasional power-selling between utilities. A small number of terrorists, attacking key power facilities when one or more engineers have infiltrated the power control centers, could wreak havoc.

Equipping likely targets with containers (i.e., bags) of pig lard has been utilized to discourage attacks by suicide bombers. The technique was apparently used on a limited scale by British authorities in the 1940s.^[26] The approach stems from the idea that Muslims perpetrating the attack would not want to be "soiled" by the lard in the moment prior to dying. The idea has been suggested more recently as a deterrent to suicide bombings in Israel.^[27] However, the actual effectiveness of this tactic is probably limited as it is possible that a sympathetic Islamic scholar could issue a fatwa proclaiming that a suicide bomber would not be polluted by the swine products.

In North America and other continents, for a threatened or completed terrorist attack, the Incident Command System (ICS) is apt to be invoked to control the various services that may need to be involved in the response. ICS has varied levels of escalation, such as might be needed for multiple incidents in a given area (e.g., the 2005 bombings in London or the 2004 Madrid train bombings, or all the way to a National Response Plan invocation if national-level resources are needed. National response, for example, might be needed for a nuclear, biological, radiological, or large chemical attack.

Fire departments, perhaps supplemented by public works agencies, utility providers (e.g., gas, water, electricity), and heavy construction contractors, are most apt to deal with the physical consequences of an attack.

Again under an incident command model, local police can isolate the incident area, reducing confusion, and specialized police units can conduct tactical operations against terrorists, often using specialized counter-terrorist tactical units. Bringing in such units will normally involve civil or military authority beyond the local level.

Emergency medical services will triage, treat, and transport the more seriously affected victims to hospitals, which will also need to have mass casualty and triage plans in place.

Public health agencies, from local to national level, may be designated to deal with identification, and sometimes mitigation, of possible biological attacks, and sometimes chemical or radiologic contamination.

Today, many countries have special units designated to handle terrorist threats. Besides various security agencies, there are elite tactical units, also known as special mission units, whose role is to directly engage terrorists and prevent terrorist attacks. Such units perform both in preventive actions, hostage rescue and responding to on-going attacks. Countries of all sizes can have highly trained counter-terrorist teams. Tactics, techniques and procedures for manhunting are under constant development.

Most of these measures deal with terrorist attacks that affect an area, or threaten to do so. It is far harder to deal with assassination, or even reprisals on individuals, due to the short (if any) warning time and the quick exfiltration of the assassins.^[28]

These units are specially trained in tactics and are very well equipped for CQB with emphasis on stealth and performing the mission with minimal casualties. The units include take-over force (assault teams), snipers, EOD experts, dog handlers and intelligence officers. See Counter-intelligence and counter-terrorism organizations for national command, intelligence, and incident mitigation.

The majority of counter-terrorism operations at the tactical level, are conducted by state, federal and national law enforcement agencies or intelligence agencies. In some countries, the military may be called in as a last resort. Obviously, for countries whose military are legally permitted to conduct police operations, this is a non-issue, and such counter-terrorism operations are conducted by their military.

See counter-intelligence for command, intelligence and warning, and incident mitigation aspects of counter-terror.

Some counterterrorist actions of the 20th and 21st century are listed below. See list of hostage crises for a more extended list, including hostage-taking that did not end violently.

The scope for Anti-terrorism systems is very large in physical terms (long borders, vast areas, high traffic volumes in busy cities, etc.) as well as in other dimensions, such as type and degree of terrorism threat, political and diplomatic ramifications, and legal issues. In this environment, the development of a persistent Anti-terrorism protection system is a daunting task. Such a system should bring together diverse state-of-the-art technologies to enable persistent intelligence, surveillance and reconnaissance missions, and enable potential actions. Designing such a system-of-systems comprises a major technological project.

A particular design problem for this system is that it will face many uncertainties in the future. The threat of terrorism may increase, decrease or remain the same, the type of terrorism and location are difficult to predict, and there are technological uncertainties. Yet we want to design a terrorism system conceived and designed today in order to prevent acts of terrorism for a decade or more. A potential solution is to incorporate flexibility into system design for the reason that the flexibility embedded can be exercised in future as uncertainty unfolds and updated information arrives. And the design and valuation of a protection system should not be based on a single scenario, but an array of scenarios. Flexibility can be incorporated in the design of the terrorism system in the form of options that can be exercised in the future when new information is available. Using these 'real options' will create a flexible Anti-terrorism system that is able to cope with new requirements that may arise.^[32]

While some countries with longstanding terrorism problems, such as Israel, have law enforcement agencies primarily designed to prevent and respond to terror attacks,^[33] in other nations, counter-terrorism is a relatively more recent objective of civilian police and law enforcement agencies.^[34][35]

While some civil-libertarians and criminal justice scholars have called-out efforts of law enforcement agencies to combat terrorism as futile and expensive^[36] or as threats to civil liberties,^[36] other scholars have begun describing and analyzing the most important dimensions of the policing of terrorism as an important dimension of counterterrorism, especially in the post-9/11 era, and have argued how police institutions view terrorism as a matter of crime control.^[34] Such analyses bring out the civilian police role in counterterrorism next to the military model of a 'war on terror'.^[37]

Pursuant to passage of the Homeland Security Act of 2002, federal, state, and local law enforcement agencies began to systemically reorganize.^[38][39] Two primary federal agencies (the Department of Justice (DOJ) and the Department of Homeland Security (DHS)) house most of the federal agencies that are prepared to combat domestic and international terrorist attacks. These include the Border Patrol, the Secret Service, the Coast Guard and the FBI.

Following suit from federal changes pursuant to 9/11, however, most state and local law enforcement agencies began to include a commitment to "fighting terrorism" in their mission statements.^[40][41] Local agencies began to establish more patterned lines of communication with federal agencies. Some scholars have doubted the ability of local police to help in the war on terror and suggest their limited manpower is still best utilized by engaging community and targeting street crimes.^[42]

While counter-terror measures (most notably heightened airport security, immigrant profiling^[43] and border patrol) have been adapted during the last decade, to enhance counter-terror in law enforcement, there have been remarkable limitations to assessing the actual utility/effectiveness of law enforcement practices that are ostensibly preventative.^[44] Thus, while sweeping changes in counter-terrorism rhetoric redefined most American post 9/11 law enforcement agencies in theory, it is hard to assess how well such hyperbole has translated into practice.

In intelligence-led policing(ILP) efforts, the most quantitatively amenable starting point for measuring the effectiveness of any policing strategy (i.e.: Neighborhood Watch, Gun Abatement, Foot Patrols, etc.) is usually to assess total financial costs against clearance rates or arrest rates. Since terrorism is such a rare event phenomena,^[45] measuring arrests or clearance rates would be a non-generalizable and ineffective way to test enforcement policy effectiveness. Another methodological problem in assessing counter-terrorism efforts in law enforcement hinges on finding operational measures for key concepts in the study of homeland security. Both terrorism and homeland security are relatively new concepts for criminologists, and academicians have yet to agree on the matter of how to properly define these ideas in a way that is accessible.

• indicates military organization allowed to operate domestically.

Given the nature of operational counter-terrorism tasks national military organizations do not generally have dedicated units whose sole responsibility is the prosecution of these tasks. Instead the counter-terrorism function is an element of the role, allowing flexibility in their employment, with operations being undertaken in the domestic or international context.

In some cases the legal framework within which they operate prohibits military units conducting operations in the domestic arena; United States Department of Defense policy, based on the Posse Comitatus Act, forbids domestic counter-terrorism operations by the U.S. military. Units allocated some operational counter-terrorism task are frequently Special Forces or similar assets.

In cases where military organisations do operate in the domestic context some form of formal handover from the law enforcement community is regularly required, to ensure adherence to the legislative framework and limitations. such as the Iranian Embassy Siege, the British police formally turned responsibility over to the Special Air Service when the situation went beyond police capabilities.