I'm of the party of Tony Blair (prime Minster, UK) towards this war. With President Bush and in today's world, confrontation with Iraq was inevitable. His position also has equivalents in the U.S. Congress but I care more about executives, so I'm citing him.
The possible outcomes were:
1) Containment that prevents Iraq from getting better weapons of mass destruction
2) Regime change that brings to power a less ambitious dictator but avoids war
3) Occupations and democratization.
Were I in Blair's shoes I would have made similar choices and would have ended up in a similarly bad position. Were I in Bush's shoes a hell of a lot would be different but I'm not, nor is anyone competent at grand strategy sadly.
Blair tried his damnedest to get option 1) to work. He was met with heavy opposition by the French government. A terrific summary of French foreign policy in this matter is "[paraphrasing] Given the choice between an Iraq that isn't complying and war, we just inspections." This is essentially choosing to allow disarmament to fail. For disarmament to succeed the statement must be "Given the choice between an Iraq that isn't complying and war, we choose war. We are not yet at that point, and so as Iraq is not on the cusp of acquiring weapons of mass destruction we wish to pursue further enhanced inspections to ideally prevent us from reaching that point." Their choice of the former was reinforced by the assurance of a veto even of compromise provisions. The United States and the U.K. alone can not prevent Iraqi acquisition of weapons via means short of war indefinitely. Multilateral support is necessary, and if you look at conditions before Bush's big push to the U.N., it has been utterly lacking.
Now if we had a president more competent than push could we have succeeded at option 1)? Maybe. Saddam Hussein has consistently been a wild card and it is not possible to really predict when he would back down. Admittedly a more effective President quite probably could have either prevented the French from choosing anti-Americanism as their Iraq policy
By supporting Bush and driving the issue to the U.N. Blair was able to wield his influence to increase the chance for option one by a good deal, but sadly he still failed. Had he not supported Bush enough to buy some influence, I'd say the probability of reaching option one drops by at least a third.
2) We haven't pursued this option. I'm willing to say that's for the best. There's a damn good chance that regime change results in Iraqi descent into civil war. There is no reliable source of information I am aware of that can credibly predict whether such a war could be avoided. On the whole, I believe that makes leaving Iraq with soft-authoritarian (which I'm not fond of in and of itself) a less humane options than outright war.
3) War, occupation, and work damn hard for democracy.
The question here is can Bush be trusted? I'm willing to believe him on his intentions to rebuild Iraq but I don't believe he's willing to take substantial political risk to guarantee that it will succeed. Fortunately I believe there are still enough leaders left in America that it will not be allowed to fail. It's harder to imagine a greater diplomatic debacle for the U.S. than failing through lack of sufficient resolve. Bush will win the war, but it is up to real leaders, like Blair and some in congress, to win the peace. Democracy in Iraq can potentially be transformative, in addition to work on the Israeli-Paliestian conflict (which has started again, finally, probably thanks to Blair) has the greatest in terms of U.S. action to bring democracy to the middle east. The greatest chance we don't control is the democratization of Iran, and I'll be calling for Bush's impeachment if he makes near as much of a botch of that as he has of Iraq diplomacy.
Ultimately, I'm gonna sleep without Saddam Hussein or even a soft-autocrat in charge of Iraq. Let alone knowing that said leader won't have access to weapons of mass destruction. It'll also help that most U.S. troops are out of Saudi Arabia and thus all that much farther from Mecca. But most important, a rebuilt democratic Iraq will be a proving grounds for idealism in American policy. I have enough faith in current non-Bush leadership, liberal democracy, and America to think we'll pass the test. Although if I'm wrong,or if Bush actually is just straight out evil, the price will be terrible. Here's hoping I'm right.