Bruce Hoffman fears that the Tokyo gas attack could herald the start of a lethal era of religious terrorism. Last month's deadly nerve gas attack on the Tokyo underground marks an historical watershed in terrorist tactics and weaponry.

Previously, terrorists had shown an aversion to the esoteric weapons of mass destruction popularised in fictional thrillers or films. Radical in their politics, most terrorists were equally conservative in their operations. Indeed, from the time of the late 19th-century Russian anarchists and the Victorian-era Fenians, terrorists have continued to rely almost exclusively on the same two weapons; the gun and the bomb. The sarin-induced deaths of at least ten commuters and the injuries to 5,000 others may have changed that for ever.

Although German, Italian, and Palestinian terrorist groups admittedly had toyed with the idea of using such lethally indiscriminate weapons, none had crossed the critical psychological threshold of actually implementing their heinous daydreams. What innovation did occur was mostly in concealing and detonating explosive devices, not in the terrorists' tactics or use of chemical, biological, or even crude nuclear weapons. Like most people, terrorists appeared to fear powerful contaminants and toxics they knew little about and were uncertain how to fabricate and safely handle, much less deploy and disperse.

The Aum Shinrikyo sect's alleged use of sarin, however, demonstrates that it is possible to execute a chemical terrorist attack and has raised the stakes for terrorists everywhere. Indeed, the pattern of terrorism over the past three decades suggests that many groups are impelled by an organisational imperative towards escalation. Believing that the public and media have become inured to the unending spiral of terrorist violence, they feel pushed to undertake more spectacular lethal deeds to achieve the same effect a smaller action would have had ten or 15 years ago. Accordingly, terrorist groups throughout the world may feel driven to emulate or create their own version of the Tokyo incident in order to attract attention to their causes.

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The alleged identity of the Tokyo attack's perpetrators is equally significant. In the past, terrorist groups were recognisable primarily as small bands of individuals belonging to an organisation with a defined set of political, social or economic objectives. However disagreeable, their ideology and intentions were at least comprehensible - albeit politically radical and personally fanatical.

The Aum sect (if responsible), however, represents a new terrorist threat. Indeed, one of the distinguishing features of international terrorism during the past 15 years has been the proliferation of groups that are predominantly religious in character and motivation and which embrace terrorism as a vehicle to realise their aims. Today, for example, nearly a quarter of the world's 50 or so identifiable terrorist groups have a salient religious motivation: a six-fold increase from the 1980 figure.

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The implications of such terrorism can be seen in the violent record of the Shi'a Islamic groups. Although they have committed only 8 per cent of all international terrorist incidents since 1982, they are responsible for nearly 30 per cent of those killed.

Moreover, as the Tokyo attacks seem to demonstrate, terrorism motivated by religion is not a phenomenon restricted either to Islamic groups. During the past decade, for example, religious terrorists or members of various cults in the United States and Israel have repeatedly come closest to using weapons of mass destruction. In 1984, Christian white supremacists plotted to poison the populations of Washington DC and Chicago by dumping cyanide in their reservoirs. That same year, followers of the Baghwan Shree Rajneesh contaminated the salad bars of restaurants in a small Oregon town with salmonella bacteria to debilitate the locals; while in Israel, messianic Jewish fanatics plotted to blow up a Muslim religious shrine in the hope of provoking a cataclysmic "holy war".

That religiously motivated terrorists can contemplate such massive acts of death and destruction reflects their belief that violence is a divine duty. Terrorism thus assumes a transcendental dimension, and its perpetrators are unconstrained by the political, moral, or practical constraints that affect other terrorists. Whereas secular terrorists generally consider indiscriminate violence counterproductive, religious terrorists regard such violence as both morally justified and a necessary expedient for attaining their goals. They regard themselves as outsiders and this sense of alienation enables them to contemplate far more destructive and deadly types of operation than their secular peers.

Finally, as the Aum sect's millenialist beliefs suggest, we may be on the cusp of a new and potentially more dangerous era of terrorism as the year 2000 approaches. One cannot predict the effect that this symbolic watershed might have on religious groups who may feel impelled to implement Armageddon with weapons of mass destruction.

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Traditional approaches and policies in countering terrorism may not be relevant, much less effective, to the threat posed by religious terrorists. Political concessions, financial rewards, amnesties and other personal inducements would likely be impractical given the religious terrorists' fundamentally alienated and polarised world view. Instead, a two-pronged course of action is needed.

On the one hand, the profound sense of alienation and isolation of these cults and religious movements needs to be counteracted. A bridge needs to be found between mainstream society and the extremists. On the other hand, the world's intelligence agencies, which have been searching for a mission since the end of the Cold War, should be deployed to counter this new threat. It is imperative that our defences and security measures are dynamic. This is ineluctably an intelligence mission and should now be a priority.

Bruce Hoffman is director of the centre for the study of terrorism and political violence at St Andrews University.