THE NEW REPORT from the Iraq Survey Group has confirmed beyond any reasonable doubt what most people have assumed for the past year: At the time of the 2003 U.S. invasion, Iraq did not possess weapons of mass destruction, and most of its programs to produce them were dormant. In more than a year of investigation, the survey group found "no evidence to suggest concerted efforts to restart" the Iraqi nuclear weapons program that had been halted in 1991; there were "no credible indications that Baghdad resumed production of chemical munitions" after 1991; and there was "no direct evidence that Iraq, after 1996, had plans for a new BW [biological weapons] program." Iraq was secretly working on banned long-range missiles and hiding both those programs and clandestine laboratories from U.N. inspectors. But the estimates by the CIA and most other Western intelligence agencies that Iraq held large stockpiles of dangerous weapons were wrong, as was much of what President Bush said about the threat.

Mr. Bush accepted those basic facts some time ago. He appointed a commission to study why the intelligence about Iraq was so wrong; though it has not yet reported, two congressional investigations have already pointed to many failings by the intelligence agencies. The administration's culpability in ignoring uncertainties in that intelligence, in failing to ask hard questions and in publicly exaggerating flawed estimates has not been thoroughly examined. Our hope is that the independent commission and a continuing congressional probe will fill that gap in the coming months.

In the meantime the report will surely fuel the debate between Mr. Bush and Sen. John F. Kerry about whether the war should have been undertaken. The two have staked out dramatically contrasting positions, focusing on a theoretical question: If the president had known what the Iraq Survey Group now reports, would he have been right to order an invasion? Mr. Bush says he would have made the same decision; Mr. Kerry says he would not have. Yet in reality no president could have known what is known now. As long as Saddam Hussein remained in power and refused to cooperate fully with the United Nations, there could have been no certainty about his weapons. Mr. Bush had to decide whether the risks of invading outweighed those of standing pat without knowing for sure what U.S. forces would find in Iraq or what would happen once they were there.

Because Mr. Bush chose to act, we know what capabilities Iraq did -- and did not -- possess, and we've learned how difficult it is to occupy and attempt to reconstruct that country. What can't be known is what would have happened had Mr. Bush chosen not to invade. Here the new report suggests some answers. Saddam Hussein, it says, was focused on ending international sanctions, which were crumbling before the crisis began. Had he succeeded, he would have resumed production of chemical weapons and probably a nuclear program as well. Mr. Kerry suggested recently that Saddam Hussein's regime would have collapsed under the inspectors' pressure. That is one possibility; another is that it would have reemerged as a significant power in the Middle East, and as a de facto or real ally of the Islamic extremist forces with which the United States is at war.

The larger question is how, or even whether, decisions about preemptive war can be made in the absence of unambiguous intelligence. This is not hypothetical: Whoever wins November's election may face a similar dilemma. Extremist anti-American governments or terrorists may acquire weapons of mass destruction, and neither al Qaeda nor the rulers of Iran and North Korea are inclined to transparency. The case of Iraq has shown that it is possible that the intelligence on which a war decision may be based may later prove to be mostly wrong. Does that mean the president cannot act in such cases? That's a question Mr. Bush and Mr. Kerry would do well to discuss.