We have them. Terrorists might even have them. But who is prepared to use them?

By Geoffrey Lean

On paper, President Bush's war against terrorism looks like the most one-sided contest since conflict began. The United States – and the other members of the international coalition – have almost all of the most fearsome weapons on the planet.

They own nearly 22,000 nuclear weapons, enough to blow up the entire world many times over. They control the vast majority of the earth's chemical weapons, and could produce biological ones. It adds up to much the most awesome combined arsenal of mass destruction ever assembled.

Against them are a handful of terrorist groups, mostly from some of the world's poorest countries. Even the states alleged to harbour and support them have relatively puny arsenals of nuclear, chemical and biological weapons, in an early stage of development. Small wonder, perhaps, that Mr Bush sounds so confident of winning.

And yet an increasing number of sober, authoritative voices – even within the US and British governments – are quietly warning that it will not be half so easy. Indeed, some experts believe that this is a war that the US and its allies cannot win, despite the unprecedented disparity in firepower.

This is partly because of terrorism's elusive nature. Immensely powerful weapons of mass destruction are useless if you cannot find the enemy, unusable if your opponents are living in your own societies. But while the coalition has little idea of where to find the controllers and associates of the terrorists who destroyed the World Trade Centre, they showed that they knew exactly where to strike to cause the most damage.

And, as Tony Blair himself pointed out to the recalled House of Commons, the chances that terrorist groups themselves may get – or already have – their own weapons of mass destruction greatly increases their menace. "The limits on the numbers they kill and their methods of killing are not governed by morality," he said. "The limits are only practical or technical. We know that they would, if they could, go further and use chemical or biological or even nuclear weapons of mass destruction."

It is a warning that has been constantly reiterated over the past quarter of a century. Indeed, shortly before the outrages in the US, Richard Clark – Bill Clinton's top counter-terrorism official – put the likelihood of a terrorist attack on the US, using such weapons, in the next decade as "100 per cent". And yet, governments are only now beginning to realise the full seriousness of the threat.

The knowledge needed to make all three of the weapons of mass destruction is freely available in the open literature. Dr Frank Barnaby, one of the world's leading experts in the field, who is compiling a report on the threat of terrorists using them, says that none would be more difficult to make and use than the bomb that blew up the Pan Am Boeing 747 over Lockerbie in 1988.

In the most serious terrorist incidents using chemical weapons, the Japanese religious cult Aum Shinrikyo dispersed the nerve gas sarin in 1994 and 1995 in the town of Matsumoto and the Tokyo underground; 20 people died in the two incidents and more than 6,000 were injured.

Sarin, tabun soman and VX, the nerve gases likely to be used by terrorists, are all extremely toxic. Most – though not all – experts say that they are easy to make, well within the capabilities of any reasonably competent chemist. But Professor Alistair Hay, of Leeds University, points out that the Japanese incidents, and their relatively low death toll, reveals a serious difficulty: how to disperse them.

The cult, for example, put polythene bags full of sarin on the floors of railway carriages, and punctured them with the tips of umbrellas. But to be really effective the gas has to be dispersed in a fine aerosol, small enough to penetrate deep into the lungs. This, says Dr Barnaby, could be done by spraying it into air-conditioning systems or, like fertilisers and pesticides, from a plane.

Biological weapons can be spread by infection, and are easier to disperse: they could, for example, be put in a city's water supply. But they can be unstable and unpredictable and cannot be stockpiled.

Experts say that anthrax is the most reliable, but it is not particularly effective. Smallpox would be much more devastating – if terrorists can get the virus from one of the two places it is still kept, in Russia. The effects would be devastating, but would only develop over weeks, whereas, until now, terrorists have wanted to make a spectacular impact. But the weapons might well be used in a long- drawn-out war.

A small nuclear device, albeit crude, would be far less destructive than a modern nuclear warhead, but could be used by a terrorist, who could carry it, undetected, in a briefcase. Even if it did not go off properly, it would scatter plutonium over a large area, making it uninhabitable. A study by the Cabinet Office has shown that such a device could devastate the centre of a city, killing people instantly within a kilometre and causing delayed cancers up to 50 miles away.

The main problem, until now, has been getting hold of the plutonium or highly enriched uranium needed for the bombs. But the collapse of the Soviet Union has led to a black market. The CIA has identified 12 groups – including Osama bin Laden's – that have been trying to buy the materials.

There were 32 seizures of illegal radioactive materials in Europe in the first half of this year. Ready-made portable "knapsack bombs" – more powerful than anything a terrorist group could make – may have already have gone missing from Russian arsenals.

For years governments, and some authorities, have argued that terrorists would never be sophisticated or well organised enough to make or use weapons of mass destruction. Over the past two weeks, most have been rapidly revising their opinions.

Dr Barnaby says that making the weapons is so straightforward that it is not so much a matter of asking "Which terrorist groups could get them?" as "Which can't?" And the events of the past two weeks have shown that they do not even need to resort to them to cause horrendous destruction: the terrorists who hijacked the planes that destroyed the trade centre were armed with nothing more awesome than knives.