If the UN is involved in a war, the participating countries don't have to pay all the costs, nor do they only send their soldiers to the front, nor are they sole responsible for what happens. And the participating countries have obligations towards the UN.

Bush Jr, by having his coalition of the willing, was exposed to their "unwillingness" to stay (elections occure!) and incapacity to force them to stay. No treaty was binding the willing countries to the US, it seems to me.

Bush pretends the world is safer without Saddam.

The secular Iraq under Saddam was a buffer against religious extremism.
Saddam was no threat to other countries, but as the ruthless dictator, managed to reign in his country, using fear and repression.
What was going on in Iraq, as long as it didn't threaten other countries, was none of our business.

Bush followed none of this, was too arrogant to prevent war (he may well have wanted it anyway) and now, his legacy won't make him very popular and not just around the world.