The fiercest debates about war usually take place after the slaughter has begun, and sometimes only when it's over. Vietnam crept up on Kennedy, and then Johnson, and even in 1965 when their private deliberations concluded with American military intervention, the public argument was nugatory. The mainstream media gave it near-total support. Somewhat later, opinion turned, and war fury both ways dominated the whole of politics. By then it was too late to save anyone from catastrophe.

The case of modern Iraq is different. We can see war coming. A better analogy is the second world war, which was preceded by epic argument between appeasers and warriors here, and then by pro- and anti-war debates in America both before and after Pearl Harbor. More telling still is George Bush I's Gulf war against Saddam Hussein in 1991. This too had a long build-up. In the course of it, the American debate was troubled. Despite the obvious pretext supplied by Saddam's invasion of Kuwait, the Senate approved military force by only 52 votes to 47. Among those who voted against Operation Desert Storm was Senator Sam Nunn, the hard-nosed chairman of the armed services committee.

George Bush II's Iraq war is being prepared in different circumstances. The political build-up is intense and almost unchallenged. There are virtually no naysayers in mainstream Washington, though they have plenty of opportunity to speak. Last week the US seized on Iraq's rejection of a new team of UN weapons inspectors, and with electric speed cast aside Kofi Annan's offer of mediation. A detailed battle plan was leaked to the New York Times, evidently by someone who thought it wasn't good enough. The momentum towards war is palpable, and to anyone who read Bush's speeches since he became a national politician it should not be surprising.

The absence of challenge can also be seen in Europe. The other day Jon Snow pressed Tony Blair on this point, and extracted an extraordinary response. Could you foresee yourself committing British troops to a ground war in Iraq, Snow asked. "I suggest we have that discussion when the decisions are actually about to be taken," Blair replied. In other words, when the discussion can influence nothing. Blair's suggested timing is precisely wrong. Any serious debate taking place after Washington has decided where it's about to go can only be destructive to the alliance. The time when a European argument might be useful is now, before the stone is set.

For a start, it would put salutary pressure on the Europeans. European nations need to formulate some positions or, ideally, one position. What do they propose to do about Saddam? How do they think about his weapons of mass destruction? What is their view on the balance between terror and freedom? How do they propose to counter the virulent American voices which remark that Europe is simply failing to address the threats it faces from terrorist-harboring states? Europe slinks silent in the shadows of this crucial discussion.

But in the absence of any real discussion in America, a more public European debate could perform an urgent service to the world. Major European leaders should be asking these questions in public places, even if one makes the charitable assumption that none of the issues are lost on the American actors. They fall into two categories. Might war work? And is it right?

There's more to hear, for example, about some of the working premises of the warriors. Are they banking on Saddam caving in fast, and his armies disintegrating? Since Washington is signaling skepticism about either an internal coup or an effective local military force of dissidents, the more likely plan envisages some 250,000 US troops, with Britain and few others possibly alongside. Have they worked out the scale of the collateral damage likely in such a big war? "No one can say whether a war will last five days, five weeks or five months," the skeptical Nunn said in 1991. With a smaller alliance to depend on, and a more tenuous casus belli , the question is more urgently worth asking today.

What, then, of the region? Is it the case that Washington, after paying lip-service to a Middle East peace process, will use the failure of its own one-sided approach as justification for the real agenda of the Pentagon hawks, who've been thirsting to devour Saddam ever since Bush I failed to do so? If Saddam is dethroned but not destroyed, what then? How deeply have the consequences of any failure in this enterprise been considered? Are such questions inappropriate for mainstream European politicians to ask or, instead, an essential contribution from allies who will be expected to go along with anything that eventually happens?

Just as pressing are the issues of justice and proportionality. Even if one accepts the contested claim that Saddam has deliverable weapons of mass destruction (WMD), how valid are the scenarios that predicate their use? If he can reach Israel, why would he dare to try if he knew the entire world would then respond in kind? Some say the most convincing explanation of his WMD is, as Tariq Aziz once confided, to prepare for revenge against Iran. It seems at least possible that the only scenario in which these weapons become a threat to America and her allies is in some last-ditch act of desperation as the Iraqi tyrant faces the build-up of an invasion army. Is it not time such perversity broke through the wall of acquiescent silence, and was coolly evaluated by informed public people in the public realm?

More largely, how will the envisaged campaign fit into the frame of international law? How stale is the thread of old UN resolutions that America - and Britain - seem determined not to try to refresh? Where might the unfolding of a long and bloody campaign fit into the doctrines of morality? That question could be as inflammatory as it was at Suez, among both officers and men. John Major, in his autobiography, recalls summoning his Anglican archbishop and Catholic cardinal to secure their approval for the Gulf war. They "gave me their public and private support", he writes, "and in so doing their reassurance that this would be a just war". The same blessing will be far harder to buy in 2003, even if the issues have been properly explored beforehand.

Germany is precluded from such discussion until and perhaps after the September election. France, on anything to do with the Middle East, has no credence in Washington. But Britain has a position and a special voice, and now is the time to make use of them, before these severe anxieties are buried under the juggernaut of a son's revenge for what happened to his father. To say that they're only worth discussing "when the decisions are actually about to be taken" is to say that everything must be left to the leaders. If these then suffer the fate of Lyndon B Johnson, booted out of politics for a war the people decided they didn't want, they will deserve it.

© Guardian Newspapers Limited 2002

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