As a recovering interventionist, let me emphasize the other point beyond legality: practicality. It's one thing to know that a dictator is evil and want to stop it, it is another whether we have political leaders capable of maintaining oversight and making the right decisions, the army trained for both fighting and building trust and institutions, and the public support that lasts throughout such a mission. Let me quote from something Billmon wrote prophetically on March 2, 2003 (two weeks before the war officially began), criticising Joshua Marshall (of Talking Points Memo):

Is there anything that suggests America
is the right country to overhaul an ancient culture, riddled with
religious and ethnic tensions, that got hung up on the conveyer belt
between medievalism and modernity? Us? The guys who couldn't find most
foreign countries on a map, and don't care?

And are the American people really prepared to sacrifice the blood and treasure it would take to try?