The gains for the US by invading Iraq was removing an open enemy of the United States who controlled a significant % of proven oil reserves and therefore had a massive long term economic capability to do harm.
That is the only reason the United States needs. Period.
Everything else is fluff/PR.
After 9/11, open enemies of the United States who have significant economic resources needed to be removed.
Iraq was chosen to be first because it was the most politically feasible to go after. Him having WMDs or whatever merely provided casus belli to do so.
If I were emperor, I would have immediately followed up by removing the regimes of Syria and Iran.
I do not consider it particularly important whether Iraq has a "stable democratic" government. I understand the strategy and think it's viable but only if the costs are reasonable which is looking like it's not.
The ultimate strawman in these debates is that if Iraq falls into civil war that somehow the United States has "lost". I guess I don't see how Iraq's stability is really our problem when it comes right down to it.
The military is very good at blowing things up, breaking things, and killing people. That's what they do. How creating a stable democracy somehow became part of "the mission" is a mystery to me.
I really don't understand why we haven't eliminated Syria's regime at this point as well as Iran's. Sure, it would plunge "the region" into chaos but there are other players in the region who have a much bigger stake at the region being stable than we do and would probably create an incentive for them to be more cooperative.
Bush doesn't seem to get this which is why he has such a low approval rating. He loses the 30% of the population who would have been against going in no matter what. He loses another 30% of the people who want more decisive action taken. And keeps part of the remaining gropu that doesn't really care that much.