A week before the second anniversary of the London bombs of 7 July 2005, the city had another taste of the fear terrorism can inflict with the discovery on 29 June 2007 of two Mercedes cars packed with gas cylinders, petrol and nails parked in city-centre locations. The failure of this attempt to inflict great loss of life was followed by another unsuccessful attack at Glasgow airport the next day, also using a Mercedes that drove at speed towards the airport entrance but became stuck in the attempt to enter the building.

The second narrow escape from mass slaughter at the hands of - it appears - a radical Islamist cell is an uncomfortable reminder of the continuing potency of this ideology and the of the motivation of its adherents. The politics and background of the members of this group responsible continue to be intensively discussed in the media, in particular the nature of any links with the wider al-Qaida movement. Yet alongside this concern, the promiscuous technology used in the attempted operation is significant in its own right. The London and Glasgow incidents reveal also what might be called the continuing globalisation of terror tactics in the first decade of the 21st century, in which the car-bomb has come to acquire a prominent place.

Sajid Huq is a teaching fellow at Columbia University and adjunct professor at New School University, New York, in 2007-08. He contributes to the blogs Addafication and The Progressive BangladeshBeirut, Baghdad, beyond

The security agencies of states fighting the "war on terror" characterise car-bombs as "vehicle-borne improvised explosive devices" (VBIED), a key tactic in what the Pentagon calls "fourth-generation" or "open-source" warfare. Indeed, car-bombs can be very powerful weapons that can destroy entire buildings, even (if several are detonated together) entire cityscapes; but their political and media impact is even greater than the physical.

In his excellent book on the phenomenon, Buda's Wagon: A Brief History of the Car Bomb, the historian Mike Davis traces the history of the car-bomb: from the use of a horse-drawn vehicle as a weapon by an Italian anarchist (Mario Buda) that killed forty people and injured 400 in New York's financial district in 1920; through the ultra-right Stern Gang's truck-attack on a British police station in Palestine in Haifa, Palestine in January 1947 that killed four and injured forty-seven; to the era of "national-liberation" wars in Vietnam, Algeria and Northern Ireland. But a key moment in establishing the effectiveness of this tactic - one which points forward to a new era rather than backwards to an old - was in Lebanon on 23 October 1983.

Also in openDemocracy on terrorism, its nature and impacts:

Fred Halliday, "Terrorism in historical perspective" (22 April 2004)

Fred Halliday, "Terrorism and world politics: conditions and prospects" (18 January 2005)

Fred Halliday, "Terrorism and its consequences: a tale of three cities" (16 March 2005)

Turi Munthe, "Terrorism: not who but why?" (21 July 2005)

Fred Halliday, "Terrorism and delusion" (12 April 2006)

Charles Townshend, "Terrorism: in search of the definite article" (3 July 2007)

See too the many articles in openDemocracy's "democracy & terror" category, and the daily security briefs, debates and articles in our focused sub-domain, terrorism. openDemocracy.net

The truck-bomb that obliterated the United States marine barracks at Beirut international airport arguably surpassed in lethal effectivity the combined firepower of the bombers and battleships of the US sixth fleet stationed off the coast of Lebanon at the time. The operation, which killed 241 marines, came six months after the truck-bomb that hit the US embassy on 18 April 1983 that killed sixty-three people (including the CIA's national-intelligence officer for the near east, Robert C Ames). But it is not just the huge death-toll that makes it a landmark in the history of urban terrorism.

Mike Davis argues that the event's geopolitical repercussions were more severe even than the loss of Saigon in 1975, on the grounds that while the latter belonged to a bygone era of bipolar rivalry, the group responsible - Hizbollah - here etched the blueprint for asymmetric warfare that augured the generation to come (see Mike Davis, "The Poor Man's Air Force", Truthout, 11 April 2006).

Hizbollah, formed in the wake of the Israeli invasion of Lebanon in 1982 by the coalescing of a number of Islamic groups (including pro-Khomeini elements nurtured by the Iranian pasdaran [Revolutionary Guards]), was long regarded by many as a tentacle of Iran's Islamic revolution. Until the 1983 attacks, the then Ronald Reagan administration, the CIA, and the French intelligence services (representing another target of the group) had paid surprisingly little attention to Hizbollah's emergence. So the pick-up truck that carried 12,000 pounds of explosives through Beirut's traffic and into the US marine compound was a devastating announcement of Hizbollah's arrival and of a form of warfare that - by precipitating the ignominious retreat of US forces from Lebanon - showed its capacity to affect the policy of even the most powerful of governments.

CIA agent Robert Baer described the scene: "The USS Guadalcanal, anchored five miles off the coast, shuddered from the tremors. At ground zero, the centre of the seven-story embassy lifted up hundreds of feet into the air, remained suspended for what seemed an eternity, and then collapsed in a cloud of dust, people, splintered furniture, and paper."

If this annihilation of a western military outpost in the middle east helped to reshape the history of urban guerrilla warfare and terror tactics, the impact can be seen most sharply in Iraq today, where the number of car-bombs - directed against US forces but also innocent civilians in shopping areas and members of the sectarian "enemy" - fluctuates but remains consistently high. Mike Davis estimates that the number of car-bombs detonated in Iraq between June 2003 and June 2006 at 578. He also argues that the very social geography that made the invasion so easy for American forces - a largely desert landscape of heavily urbanised oases connected by a complex highway system - makes it a perfect playground for car-bombers (also often suicide-bombers).

Beyond Iraq, vehicular bombs punctuate a new age of globalised urban warfare which utilise the advantage for the weaker party of "asymmetric warfare". A lethal, inexpensive weapon can inflict great damage and act as an enormous psychological boost to the perpetrating group; it also closes the "asymmetry" of the struggle in perception, if not in reality. The propaganda effect here is of inestimable value to insurgents, for car-bombs are also quintessential terrorist "spectacle" - seducing an image-hungry media, scandalising the public, provoking politicians.

The car-bomb is a defining tactic of the era of global urban terrorism. Just as political assassination and suicide-bombing have spread by example and technology-transfer across the world, so the car-bomb is becoming less and less a monopoly of any particular group - notwithstanding the fact that certain networks (at present, al-Qaida or Sunni insurgents in Iraq) seem to "brand" it as one of their special tactics. Even where it doesn't work, as in London and Glasgow on 29-30 June 2007, it still frightens. There may be only the smallest of chances of any individual citizen falling victim to this anonymous threat, but its symbolic power has everyone in its grip.