War, to me, is only acceptable in the two situations Rich posted earlier: self defense and in defense of others (to stop an ongoing genocide, in other words). But there needs to be overwhelming evidence of ongoing genocide before I would support an invasion. Nobody appointed us the world's policemen, and we shouldn't be acting like it unless we see something that absolutely must be addressed in the name of humanity as a whole.
Also, please don't confuse "pre-emptive" war with "preventative" war. Both terms basically mean that you attack the enemy before they can attack you. The difference lies in the enemy. If that enemy is in the process of preparing to attack your country, then it is a "pre-emptive" war, and would fully qualify, in my opinion, as self defense. But if that enemy is not in that process, but you merely suspect that they may be, sometime down the road...that is "preventative" war.
If Iraq had actually had all the things we were told they had, then this might be labled a "pre-emptive" war. However, since we didn't, and the story has shifted away from how Iraq was preparing to attack us and our allies, it's become clear that what we are involved in is actually a preventatve war. We attacked because Iraq, "might, someday, be able to launch an attack against the United States." Things shifted from "weapons of mass destruction" to "WMD programs", with little evidence of either.
A pre-emptive war is a defensive war. A preventative war is offensive. There-in lies the difference.