War is bad, but the other options were not good
Worldwide, there is much opposition to the Iraqi war. It has been said many times that Saddam was not a serious enough threat to warrant an invasion. It is often not mentioned that a major reason why Saddam wasn't an imminent threat was that harsh measures were imposed upon Iraq because of the Kuwait invasion and the subsequent failure to participate in weapons inspections, and that these measures have serious consequences.
There were 3 main options for Iraq's future. Neither of them is pleasant.
--Sanctions
--Saddam with no sanctions
--War
It's "easy" to be against war, but are you in favor of sanctions or an unfettered Saddam regime?
Along those lines, I would argue that in order to credibly oppose the war, one must disagree with at least one of the 3 following premises:
1. The sanctions against Iraq needed to end.
2. A Saddam regime without sanctions would pose an unacceptable security risk.
3. The only way to remove Saddam from power was the use of military force.