Too costly. If it'd be too costly to invade Imperial Japan, it'd certainly be too costly to invade Iran. Too bad Truman's not here to make the hard call.
Patton and MacArthur wouldn't have much of a problem (neither would Pershing, NOR Teddy Roosevelt.)
I spent a year in Kosovo dealing with Muslims. I'm not sure that your everyday American understands how they think. At the very least they will try to out reproduce us. It is a fight I don't think we are ready for.