The reason it wasn't a quick in-n-out was because a "significant enough to cause political problems" chunk of the American public would have never tolerated wrecking a nation purely as punishment and then leaving.

Wanna blame someone? Look around. HN is chock full of exactly the kind of people who cause this outcome (albeit many of them are too young to bear any responsibility). Upper middle class types, the kind of people who vote, write the occasional blog, people who's voices are heard politically and who generally will not tolerate violence without some pretext of necessity. They're why we got stuck in Afghanistan. Lower class voters would have mostly been fine with "bomb them back to the stone age and go home". We had to spend 20yr pretending we were actually going to fix things so that educated voters could feel good about themselves. It's like the nation state equivalent of not wanting to sell a project car you'll inevitably never get around to and then finally selling it because you're moving and the prospect of incurring a bunch of cost hanging onto it finally forces you to deal with reality.