In recent days, George W. Bush has accused those asking awkward questions about the whereabouts of Saddam Hussein's weapons of mass destruction of rewriting history. "We made it clear to the dictator of Iraq that he must disarm," Mr Bush said last week. "He chose not to do so, so we disarmed him. And I know there's a lot of revisionist history now going on, but one thing is certain. He is no longer a threat to the free world and the people of Iraq are free."
But if anyone is revising history, it is the US president. Iraq's WMD programme was the test case for Mr Bush's doctrine of pre-emption. The Iraqi threat was "grave and growing", Mr Bush declared. "Intelligence gathered by this and other governments leaves no doubt that the Iraqi regime continues to possess and conceal some of the most lethal weapons ever devised," he warned on the eve of war.
"We know where they are," said Donald Rumsfeld, the defence secretary, as US and British forces advanced into Iraq. But 90 days after the main fighting ended, US forces have yet to find a single chemical artillery shell, litre of anthrax or uranium enrichment facility. Lieutenant-General James Conway, the senior US marine in the Iraq region, has explained the failure to find any weapons by concluding that "we were simply wrong".
Yet rather than asking why US intelligence was wrong, Mr Bush now claims that the war was about freeing the Iraqi people. No doubt Iraqis are better off without Mr Hussein. But even a staunch supporter of the Iraq war such as Paul Wolfowitz, the deputy defence secretary, has agreed that Mr Hussein's misrule "by itself (was) not a reason to put American kids' lives at risk".
Mr Bush's impatience with those who want to know why his forces have not found WMD is shortsighted. In a democracy, it matters whether the people can believe what their leaders tell them. If the facts on the ground do not match what leaders say, the consequences can be profound. The Vietnam war showed how difficult it is to close the credibility gap once it has opened up.
The American people believed what Mr Bush and his advisers told them about the threat, especially after a forceful presentation by Colin Powell, secretary of state, at the United Nations. But there are disturbing signs that the administration may have exag gerated its case and hidden information that would have weakened the urgency of going to war. Warnings of an incipient nuclear threat - Richard Cheney, vice-president, spoke of Iraq having "reconstituted its nuclear weapons" - appear to have been based on disputed evidence and documents known to have been forged.
We have also recently learnt that the Defence Intelligence Agency concluded in November that Mr Hussein would not resort to WMD unless his survival was at stake - exactly what critics of a rush to war were arguing.
Getting to the bottom of this issue matters, finally, for the centrepiece of Mr Bush's foreign policy - the doctrine of pre-emption. This doctrine rests on knowing the intentions and capabilities of those the US might pre-empt. But if it turns out that the intelligence on Iraq was flawed - or, worse, deliberately exaggerated - America's ability to pre-empt future, and perhaps graver, threats will be harmed. Given the political storm that has buffeted Tony Blair, a future British prime minister will think twice about joining the US in another pre-emptive strike. And once serious doubts about the credibility of what America's leaders say about new threats are raised, the American public may not want to go along either.
We must get to the bottom of why US intelligence was so far off the mark on Iraq's WMD stocks. Congress may hold hearings but its work will inevitably be compromised by partisan squabbling. The president would do himself and the country a favour if he pre-empted Congress by appointing an independent commission of people with unquestioned integrity to review the intelligence. Modelled perhaps on the Tower commission that President Ronald Reagan appointed after the Iran-Contra scandal, the commission would determine what we knew and when we knew it. And it would recommend how to gather, analyse and disseminate intelligence in ways that reassure rather than mislead.
What happened to Iraq's weapons of mass destruction - whether they were hidden, given to others, or destroyed - is too important a question to be left to the vagaries of partisan politics. It must be answered soon, by those with the credibility to resolve the matter once and for all.
The writers, senior fellows at the Brookings Institution, are co-authors of America Unbound: The Bush Revolution in Foreign Policy, to be published later this year