Basically, I think there are two main reasons to go to war:
1. U.S. prestige. If we fail to follow through on our stated intentions, it makes us look weak. The world won't see this as a disagreement with an administration, the United States seeing reason, or anything of the sort. They will see it as the United States backing down under foreign power and our enemies (and some of our allies) will celebrate it as a great victory.
2. The non-proliferation treaties will be worthless except as toilet paper. Our last chance at salvaging them is to convince Iraq to disarm and this will not be possible without, at the least, an imminent and convincing threat of war. If we succeed here, we can at least make progress against North Korea and Iran, but if we don't, there is no way we are going to convince them of anything.

I think that it is highly probably that, shortly after the CWC, BWC, NPA, etc. are discredited that these countries will have Nuclear, Biological, and Chemical weapons (NBC). Furthermore, I think it will destabilizing for these countries to have these weapons, which will enable them to engage in wars against their neighbors without fear of retaliation. They can also engage in genocides and other atrocities without repercussions. Usually, the mere threat of force is all that is needed and right now we protect all of our allies under what is known as the "Nuclear Umbrella." A lot of people say the United States shouldn't be the world's policeman. If we let these countries have NBC (well, they can have the broadcaster), then we CAN'T be the world's policeman, in any situation.

In essense, my argument for a war in Iraq is that a stitch in time saves nine. I believe that war should only be used as a last resort, but through rhetoric from all sides, the U.S., France, Iraq, etc., it has become necessary and to refuse to do so would have devastating consequences.

-Biscuits