How we tackle this debate is critical. How the Iraq crisis is resolved will shape future crises, for Iraq will probably be part of a series of campaigns against the "axis of evil". It is likely that Saddam does have some WMD, likely that the security council will endorse action that ends in his overthrow and likely that the war will be won quite easily. Iraq's forces were shattered and have not been rebuilt, US power is unbelievably greater.

Why then should President Bush's policy be opposed and what changes must we insist on? He summarises his policy as tackling "the worst weapons in the hands of the worst leaders". But little is being done with respect to the "worst weapons". Attempts by the international community to control nuclear, biological and chemical weapons have been relentlessly undermined by Bush's Republican party for more than a decade.

Military action against states flouting international norms on WMD can only be justified if we and the US are implementing them too. Saying "do as we say", not "do as we do", is an invitation to everyone to acquire them. Tony Blair is making terrorism and proliferation far easier by accepting Bush's deliberate introduction of anarchy in international security. Members of the Bush administration were in office in the 1980s and were silent when Iraq used poison gas on Iran, the US's arch-enemy at the time. And we in Britain may have forgotten that our airforce used poison gas to suppress rebellion in Iraq in the inter-war period; one can be sure that the Iraqis have not.

You will hear two further arguments in support of US policy. The first is: "We are democracies so our weapons are OK and we do not need further control." This is no more than saying that because we are good we cannot be bad. The second is that only western nations believe in ethics and law, so they are no good in the real world. This is as self-contradictory as the first, and insidiously racist.

Sustained by such principles, the architects of President Bush's policy hope to see it applied to Iran, North Korea and, ultimately, China. For those Republicans who pride themselves on having destroyed the Soviet Union and unified Germany, their duty now is to achieve the same success over Beijing's nuclear-armed communist dictatorship, which oppresses the Tibetans, runs its economy from a prison gulag and represses religious freedom.

Friends look at me as if I have lost the plot when I say this. But John Bolton, Richard Perle, Condoleezza Rice, Frank Gaffney and Paul Wolfowitz have no problem with a pre-emptive political-military strategy towards an emerging China. Ambassador David Smith, who contributed to the influential National Institute for Public Policy report on nuclear strategy, explained that "the US has never accepted a deterrent relationship based on mutual assured destruction with China" and will act to prevent China gaining such a capability.

Even though we were told that deterrence had stopped Saddam from using his weapons in the last Gulf war, now it is said that he cannot be deterred and must be pre-empted. Yet it is safer and easier to replace deterrence with elimination of all WMD. A policy of inspections that are militarily enforced would be quite useful if it were applied universally and provided a guarantee against one nation breaking a global ban on nuclear arms. We need to use the fact that WMD and human rights are now on the international agenda as an opportunity. The introduction of a pre-emptive strategy by Washington contradicts Nato strategy and must be rejected at the alliance's November summit.

Our immediate focus should be a precise and public debate on the terms of the cabinet discussion, in accordance with the constitutional principle of collective responsibility. We should insist that the UN mandate a conference to manage and eliminate all WMD without exception - including American and British nuclear weapons - in accordance with the existing obligations of UN member states.

If economic and other events do not deflect an attack on Iraq, there will be no declaration of war by the Commons because our constitution gives that power to the prime minister. Perhaps people should insist that parliament change the constitution, so that it appropriates the power to make war on behalf of the people. Britain would then be importing some of America's democratic, rather than its military, strength.

· Dan Plesch is senior research fellow at the Royal United Services Institute and author of Sheriff and Outlaws in the Global Village

 dplesch@rusi.org