I also don't find this to be an easy question to answer. And let me start with the necessary disclaimer that while I believed a war would be necessary to depose Saddam, I opposed both the timing and manner of the actual war as the Bush administration pursued it.
For me, there is no escaping the fact that the prewar intelligence estimates regarding Iraq's WMD programs—and particularly its nuclear program—were wrong. Iraq was not 4-5 years away from having a nuclear weapon, as I and the rest of the Clinton administration had been led to believe.
On the other hand, going back in time to 2002, but knowing that Iraq did not pose the same kind of strategic threat that we believed, I think there still would have been grounds to argue that a full-scale invasion to topple Saddam was a reasonable option.
Saddam Hussein's regime was still a source of considerable instability in one of the most important and fragile regions of the world. Setting aside the invasions of Kuwait and Iran, and the wars he threatened with Syria and Israel, his behavior throughout the 1990s (when he did not have nuclear weapons and after suffering the horrible defeat of the Gulf War) was still astonishingly aggressive, risk-tolerant, and determined to overturn the status quo. His 1993 attempt to assassinate George Bush, his 1994 threat to Kuwait, the 1996 attack on Irbil, provoking Desert Fox in 1998, and trying to move Iraqi ground forces to the Golan to provoke an Israeli military action in 2000 all speak to the problems his regime created as a matter of routine.
There was still a residual WMD threat. What we have learned since the fall of Baghdad is that Saddam remained determined to acquire these weapons at some point in the future and had preserved rudimentary elements of the programs, which he intended to use to rebuild them after the sanctions were lifted. With the exception of the missile area, these were not very active programs, and the threat from Iraqi WMD (and particularly nuclear weapons) was much, much further away than was believed, but it was not gone completely. I think this the weakest argument, but not entirely irrelevant.
There was also the human rights argument. For me, this was very compelling, although I recognized that it wasn't necessarily as important for every American. Even before the revelations of postwar Iraq, only the most obtuse failed to recognize that Saddam's regime was among the most odious of the last 50 years. As someone who supported previous U.S. humanitarian interventions in the Balkans and elsewhere—and who wished we had taken action in Rwanda—the argument was an important aspect of my own conviction. I felt guilty all throughout the 1990s that we were not doing more for the Iraqi people (especially after we betrayed them in 1991). Unfortunately, until Sept. 11, I saw no likelihood that the American people were going to support an invasion—which was the only policy that could actually relieve Iraq's misery. However, I had supported both revising the sanctions (years before the Bush administration would adopt them under the banner of "smart sanctions"), and I argued for a more aggressive covert action program in the vain hope that it might produce regime change.
Which brings me to my last point: the range of available options. In asking whether the United States should have gone to war with Iraq I think we also need to address what our alternatives would have been. We need to remember that our Iraq policy was in bad shape starting in the late 1990s. I still find the alternatives all pretty bad—although some are not necessarily as bad as I thought them before the war.
I think the war put to rest the fantasies of the neocons that we could simply arm Ahmad Chalabi and a few thousand followers (followers he still has not actually produced), give them air cover, and send them in to spark a rolling revolution. Richard Perle and others argued for that initially, but in the end they had to support a full-scale invasion as the only realistic course. The covert-action-based regime-change policies that I and others in the U.S. government had pushed for as an alternative never had a high likelihood of success, either—they were just slightly more likely to produce a coup and much less likely to create a catastrophic "Bay of Goats," as Gen. Anthony Zinni once put it. Ironically, I think the events of the last 12 months have also indicated that containment was doing both better than we believed, and worse. On the one hand, the combination of inspections and the pain inflicted by the sanctions had forced Saddam to effectively shelve his WMD ambitions, probably since around 1995-96. On the other hand, the behavior of the French, Russians, Germans, and many other members of the United Nations Security Council in the run-up to the war was final proof that they were never going to do what would have been necessary to revise and support containment so that it might have lasted for more than another year or two.
The one alternative policy that looks better in retrospect is deterrence—which was the idea that we could allow containment to collapse because we could still deter Saddam from making mischief through our own military power. While I think Saddam's astonishingly reckless behavior before the war only confirms the prevailing view among Iraq experts that this was not someone we would have wanted to trust with nuclear weapons, the postwar revelations suggest that he was so much further away from having those nuclear weapons that we might have safely opted for deterrence in the expectation that we could have found an alternative way to deal with him in the years before he did get his hands on a nuke.
If I had to write The Threatening Storm over again I certainly would not have been so unequivocal that war was going to be a necessity. However, I still would have pointed out that there was a strong case for removing Saddam's regime (for the reasons mentioned above) and that realistically the only way to remove him from power was to mount a full-scale invasion. I might have decided that when you weighed all the pros and cons, deterrence and invasion might have been roughly equal, but I would have pointed out that a key difference between them was that if you opted for invasion you were removing a great evil from the world and creating the possibility that we could turn Iraq into a real positive, as Tom and Fareed argued when they made the case on the basis of democratization. It would not have been as compelling, but my guess is that many readers would still have come to the conclusion that war was the least-bad choice among a menu of imperfect options.