25 Years After 9/11: Whatever Happened to Mass-Effect Terrorism?

9/11 changed everything. It led to two decades when the US and much of Europe became fixated on Islamism. Western strategists became fluent in half-absorbed subjects such as Wahhabism, Pashtun tribal customs, Frontier Crimes Regulations and the Anbar Awakening while giving insufficient attention to Chinese economic dominance and Russian military rearmament. Who knows what that diversion of strategic and intellectual resources (and the chaos it left behind in the region) has cost the West?

9/11 also bookends that remarkable period from the collapse of the Soviet Union in 1989/90 when peace and reconciliation seemed to break out everywhere. The Oslo Accords of 1993 appeared to chart a trajectory towards a two-state solution in Israel/Palestine. One year later the peaceful multiparty elections in South Africa were nothing short of a miracle. For years Afrikaners had insisted they would fight black rule until the last drop of white blood. This was the brief ‘unipolar moment’ under the then-still largely benign supervision of Washington.

‘Both Triumphant and Disastrous’

9/11 ushered in a new world – one with which Thomas Hobbes would be all too familiar. ‘Covenants, without the sword, are but words and of no strength to secure a man at all.’ ‘Force and fraud are in war the two cardinal virtues.’ Almost every quote from Leviathan is applicable.

Of course, first and foremost, 9/11 was a tragedy for the victims and their families. It was an act of appalling savagery. It was not only the largest terrorist attack in history by a country-mile, but it also introduced a new concept of mass-effect terrorism.

From the terrorists’ point of view, it was also an attack of breath-taking scale and ambition. Osama bin Laden had the imagination to view an aircraft full of fuel and passengers as a terror weapon by knitting together two of Al-Qaeda’s (AQ) previous failures: the truck-bomb attack on the World Trade Center in 1993 and the 1995 Operation Bojinka in the Philippines. Both attacks had been planned by Ramzi Yousef, whose uncle, Khalid Sheikh Mohammed, was the chief planner for 9/11.

It was also, from an AQ viewpoint, both triumphant and disastrous. Its success was that a superpower overreacted for two decades and allowed 9/11 to shape the whole of Western foreign policy as a Global War on Terror. Its failure was due to Washington’s largely successful determination to destroy AQ, a success which was only imperilled when President Trump approved the feckless Doha Accords with the Taliban in 2020, opening the way to the return of AQ to Afghanistan the following year. The latest UN report on terrorism confirms that AQ continues to re-establish itself.

‘Terrorist Ambitions Remained High’

As early as 1998, the British government had warned Washington that it was the likely next target in an operation which would involve hijacked aircraft. This story was told in detail by RUSI for the 20th anniversary of 9/11. The intelligence was briefed to both Presidents Clinton and Bush (junior). Contrary to widespread opinion, the concept of mass-effect terrorism had been exercising minds in Washington and Whitehall long before 9/11. If they had not made the intellectual leap of imagining airliners as weapons, they had nonetheless been thinking a great deal about mass-casualty attacks. This was prompted by three convergent factors.

First, in 1995 a doomsday cult known as Aum Shinrikyo released sarin on the Tokyo metro in an attack which (had it been carried out more effectively) could have killed thousands. This led to much discussion of how to react to a similar attack on the London Underground. There was also concern about chemical and biological substances being brought into the country inside the diplomatic mail bags of state sponsors of terrorism (the bags being protected from scrutiny by the Vienna Convention of 1961).

Second, there were fears about AQ getting its hands on nuclear material and producing a dirty bomb (not a nuclear explosion). At the time, it was believed that fissile material was being sold by former employees of the Soviet nuclear programme and finding its way through Central Asia to Pakistan. Most of it was a scam called ‘red mercury’ which was thoroughly investigated at the time, but there were enough more credible reports to create uncertainty.

Finally, it was known that AQ had two scientists working on chemical, biological and nuclear (CBN) experiments out of Derunta camp near the Afghanistan–Pakistan border. Not all the fine detail was available, but one of the scientists, Dr Sultan Bashiruddin Mahmood, had previously worked on Pakistan’s own sovereign nuclear programme. This story was well told a decade later by the veteran journalist Peter Bergen.

‘Herein Lies the Real Mystery’

Thus began a decade-long US fixation with the danger of terrorists obtaining access to Pakistan’s nuclear weapons. President Obama insisted that ‘The single biggest threat to U.S. security, both short term, medium term and long term, would be the possibility of a terrorist organization obtaining a nuclear weapon.’

Once 9/11 had happened, the assumption was that there would be numerous copycat attacks. In London, much focus was devoted to the towers at Canary Wharf, which housed some of the world’s premier financial institutions and which were also close to the flight paths of both Heathrow and London City airports.

In the years immediately after 9/11, terrorist ambitions remained high, including the Bali bombings in 2002 (202 killed), the Madrid train bombings in 2004 (193) and London 2005 (52). There were two dangerous plots foiled in the UK, Crevice in 2003 and Rhyme in 2004 but the only one on the scale of 9/11 was Operation Overt in 2006, AQ’s plan to bring down seven airliners in mid-Atlantic. Fortunately, it was thwarted by MI5.

And herein lies the real mystery. There have been numerous nasty and costly terrorist attacks since, but none on the 9/11 scale. There is a danger of sounding callous when observing that none of Mumbai in 2008 (166 killed), Paris in 2015 (130) and Sri Lanka in 2019 (269) come even close to 9/11 (2,977).

‘There the Comparison Ends’

Two types of terrorism have emerged since Overt in 2006.

The first was ‘urban siege’, ‘swarm’ or ‘rampaging’ terrorism which we first saw at Mumbai in 2008 and the Westgate shopping mall in Nairobi in 2013 (which killed 71). This is an effective form of terrorism because it captures press attention for hours (62 hours in the case of Mumbai) and makes prompt armed response extremely difficult. However, it is also costly for the terrorists, who are nearly always killed or captured, and their supporting organisations and states identified. The Indians were relentless in following up the Mumbai attack although Lashkar-e-Tayyiba still exists and represents a significant threat to regional peace.

The second (and now the most common) is ‘lone wolf’ terrorism usually committed by lone (and lonely) young men who have become self-radicalised online in their bedrooms and take the family car or a kitchen knife and seek to kill as many people as possible. The problem with this second type is that it is almost indistinguishable from wider mental health problems brought about by social isolation. It risks a substantial drain on the precious resources of security services, which need to be focusing on strategic threats (such as those posed by Russia, Iran and China as well as terrorist groups).

In terms of mass casualties, only one is comparable with 9/11: the brutal Hamas incursion into southern Israel on 7 October 2023. As a proportion of Israel’s population, it is probably the most lethal terrorist attack in history. But there the comparison ends. The attack involved guns, knives and grenades – a far cry from the use of aircraft as a weapon against a ground-based target.

'Still Too Early to Say'

Aircraft and their passengers have often been targets before, notably Pan Am Flight 103 at Lockerbie in 1988 (270 deaths), UTA Flight 772 the following year over Niger (170) and the often-forgotten Air India Flight 182 off the Irish coast in 1985 (329). But in each case the aircraft and passengers were the target rather than the means of delivery against an objective on the ground. 25 years after 9/11, one would have expected to see several similar attacks using aircraft against buildings or (even worse) against stadiums full of spectators.

Why has this not happened?

Precautions taken by airlines should take some credit. Cockpit doors have been strengthened, and protocols have been largely enforced, although there are signs of laxness creeping back in.

Security services have used bulk data to identify suspicious trends and plots early. AI will help here. It is now much harder for terrorists to operate as groups.

And the US military, its intelligence services and allies wreaked considerable damage on both AQ and Islamic State from 2001 through until 2019.

So, was 9/11 a one-off rather than, as was feared, the start of a new trend? It is still too early to say. Motivation for terrorism remains strong (especially following the destruction of Gaza since October 2023). Advances in drone technology in Ukraine since 2022 could provide terrorists with an effective means of delivery at a time when countermeasures are still in their infancy. And AI could enable terrorists to research and develop lethal payloads.

As we remember 9/11, it is still too early to welcome the absence of successful repeat during that costly quarter-century.

© RUSI, 2026.

The views expressed in this Commentary are the author's, and do not represent those of RUSI or any other institution.

For terms of use, see Website Terms and Conditions of Use.

Have an idea for a Commentary you'd like to write for us? Send a short pitch to commentaries@rusi.org and we'll get back to you if it fits into our research interests. View full guidelines for contributors.