Tomorrow will probably witness the largest political demonstration in British history. The organisers of the Stop the War march are confidently estimating that 500,000 people will attend, while privately hoping that up to twice that number may be mobilised. The crowd will be extraordinarily diverse in its character, ranging from hard-Left activists to Conservative-inclined pensioners deeply concerned about the welfare of the world which they will leave to their grandchildren. Many of the marchers will be well-meaning if misguided. For Charles Kennedy to choose to make political capital for the Liberal Democrats is far less honourable. And some of the company he will be keeping will be less than savoury. There are other ways available to him to make his point.

The point that should trouble those who are considering taking part whether they are really entirely in accord with their fellow marchers on this question. The express purpose of this event is to "Stop the War". It is not "Delay the War", "No War without United Nations blessing" or "Think about the War more carefully". Many of the platform speakers oppose removing President Saddam Hussein in all circumstances. Some will display their absolute hostility towards the United States as well as their disagreement with the current administration. Others will make their loathing of Israel unmistakable. To march is to reject the idea of armed intervention in Iraq altogether.

That the notion of military action is the cause of disquiet is neither surprising nor unreasonable. This crisis lacks some of the immediate moral clarity of other controversies. In the case of the Falkland Islands and Kuwait, sovereign territory had been seized by an outside power. The issue at hand was whether this behaviour was acceptable. The Taleban in Afghanistan openly sheltered al-Qaeda, the organisation that almost all reasonable people acknowledge was responsible for the September 11 atrocities. This alone, for most observers, legitimised a campaign to break the terrorist network and depose those who had backed them. Kosovo was more awkward, but few could deny that shameful abuses of human rights were taking place or pretend that Nato was inspired by selfish motives.

On that basis, therefore, it is perfectly fair to ask why Saddam and Iraq now constitute a clear and present danger. It is not foolish to wonder whether political tools familiar from the Cold War era — containment and deterrence — cannot be deployed again on this occasion. The suggestion that UN inspectors should be offered more time, extra resources or a stronger mandate is seductive but mistaken. The doctrine of “pre-emption”, which the Bush Administration has adopted as its rationale for war, does appear sweeping in implication, but the potential tragedies being pre-empted are on a breathtaking scale. September 11 has changed the very concept of threat and the nature of the response required to meet that threat.

The unfortunate difficulty is that containment and deterrence cannot work against biological, chemical and “dirty bomb” material as they did with nuclear missiles. This is a different world from that of the Cold War. The unique quality of these appalling weapons is that they could be used relatively simply and randomly leaving no fingerprints.

Unless clear rules are established, by force if needs be, then such poisons will become the currency of future conflict. An inspection regime, without the co-operation of its hosts, will not remove them. That the will to unleash such horror exists cannot, after September 11, be doubted. To ask politicians to “Stop the War” is a democratic right. However, it also obliges those issuing that demand to identify an alternative strategy for dealing with a danger more immediate than any of us wants to contemplate.

This article appeared as an editorial in the Times newspaper and is subject to their copyright.