A war with Iran would last maybe 2-3 months until the military was soundly defeated. The problem comes afterwards in the "nation-building and security phase" where the military become police officers and try to restore as much normalcy as possible.
If this was like WWII, these countries would have been steamrolled and the backs of their people broken until they had no will. So let us be glad we are all civilized now.
and the US can nuke the fukk out of the planet, we all know that. The problem is everyone wants to make a moral argument (see even Netanyahu's terminology here) and the argument falls apart when you start doing that. Also you're way overlooking post-WWI and post-WWII reconstruction efforts