Washington ? An antidote for grand imperial ambitions is a taste of imperial success. Swift victory in Iraq may have whetted the appetite of some Americans for further military exercises in regime change, but more than seven weeks after the president said, “Major combat operations in Iraq have ended,” combat operations, minor but lethal, continue.

And overshadowing the military achievement is the failure — so far — to find, or explain the absence of, weapons of mass destruction that were the necessary and sufficient justification for pre-emptive war. The doctrine of pre-emption — the core of the president’s foreign policy — is in jeopardy.

To govern is to choose, almost always on the basis of very imperfect information. But pre-emption presupposes the ability to know things — to know about threats with a degree of certainty not requisite for decisions less momentous than those for waging war.

Some say the war was justified even if WMDs are not found nor their destruction explained, because the world is “better off” without Saddam. Of course it is better off. But unless one is prepared to postulate a U.S. right, perhaps even a duty, to militarily dismantle any tyranny — on to Burma? — it is unacceptable to argue that Saddam’s mass graves and torture chambers suffice as retrospective justifications for pre-emptive war. Americans seem sanguine about the failure — so far — to validate the war’s premise about the threat posed by Saddam’s WMDs, but a long-term failure would unravel much of this president’s policy and rhetoric.

Saddam, forced by the defection of his son-in-law, acknowledged in the mid-1990s his possession of chemical and biological WMDs. President Clinton, British, French and German intelligence agencies and even Hans Blix (who tells the British newspaper The Guardian, “We know for sure that they did exist”) have expressed certainty about Iraq having WMDs at some point.

A vast multinational conspiracy of bad faith, using fictitious WMDs as a pretext for war, is a wildly implausible explanation of the failure to find WMDs. What is plausible? James Woolsey, President Clinton’s first CIA director, suggests the following:

As war approached, Saddam, a killer but not a fighter, was a parochial figure who had not left Iraq since 1979. He was surrounded by terrified sycophants and several Russian advisers who assured him that if Russia could not subdue Grozny in Chechnya, casualty-averse Americans would not conquer Baghdad.

Based on his experience in the 1991 Gulf War, Saddam assumed there would be a ground offensive only after prolonged bombing. U.S. forces would conquer the desert, then stop. He could manufacture civilian casualties — perhaps by blowing up some of his own hospitals — to inflame world opinion, and count on his European friends to force a halt in the war, based on his promise to open Iraq to inspections, having destroyed his WMDs on the eve of war.

Or shortly after the war began. Saddam, suggests Woolsey, was stunned when Gen. Tommy Franks began the air and ground offenses simultaneously and then “pulled a Patton,” saying, in effect, never mind my flanks, I’ll move so fast they can’t find my flanks. Saddam, Woolsey suggests, may have moved fast to destroy the material that was the justification for a war he intended to survive, and may have survived.

Such destruction need not have been a huge task. In Britain, where political discourse is far fiercer than in America, Tony Blair is being roasted about the missing WMDs by, among many others, Robin Cook, formerly his foreign secretary. Cook says: “Such weapons require substantial industrial plant and a large work force. It is inconceivable that both could have been kept concealed for the two months we have been in occupation of Iraq.”

Rubbish, says Woolsey: Chemical or biological weapons could have been manufactured with minor modifications of a fertilizer plant, or in a plant as small as a microbrewery attached to a restaurant. The 8,500 liters of anthrax that Saddam once admitted to having would weigh about 8.5 tons and would fill about half of a tractor-trailer truck. The 25,000 liters that Colin Powell cited in his U.N. speech could be concealed in two trucks — or in much less space if the anthrax were powdered.

For the president, the missing WMDs are not a political problem. Frank Luntz, a Republican pollster, says Americans are happily focused on Iraqis liberated rather than WMDs not found, so we “feel good about ourselves.”

But unless America’s foreign policy is New Age therapy to make the public feel mellow, feeling good about the consequences of an action does not obviate the need to assess the original rationale for the action. Until WMDs are found, or their absence accounted for, there is urgent explaining to be done.