Millions of people are in danger in lots of other places, we only bother to make stern condemnations, if we bother to take note at all. Why is Iraq special?
Bush also got involved in Liberia where the people were about to revolt and it looked like a bloody civil war was inevitable. Bush helped negotiate a peaceful resolution with the dictator stepping down without a bullet being fired. Obama sent troops into Libya to protect people there. He didn't in Egypt for whatever reason. But no one wants to talk about situations like these, because the world would rather practice confirmation bias that Americans are evil imperialists.
Likewise, that's not a reason to go to war. That's an excuse one can use if one already wants to go to war. It is not compelling us to war.
After WWI, the Treaty of Versailles forbid Germany from re-arming. He ignored the terms of the cease fire. If someone had intervened at some point and enforced the terms of the cease-fire, WWII might have been prevented. (Note this isn't a Godwin where someone incorrectly insists that someone else is just like Nazis. I'm merely linking to the best known example of a cease-fire after a war where a party ignored the terms and illegally pursued weapons again.)
What are you suggesting should be done when someone breaks the terms of a cease-fire and attempts to re-arm after 11 years of failed sanctions? Starve the people more?
...there really isn't any moral argument that we should prevent them from having WMD.
Other than the fact that he already attempted genocide on the Kurds? Other than the fact that he funded terrorism against Israel? Other than the fact that he openly celebrated on 9/11 and congratulated the terrorists and made it quite clear he would do the same if he was able?
Surely, there is a moral argument to prevent someone from obtaining WMD when they've already attempted genocide once. Unless you're saying genocide should always be ignored as a moral argument.