No sane president (Not even regan or bush) would use a nuke. Worse we would do is deploy some of that black project bioweapons you can bet your butt they have stored in max security facility.
But yes initially we would flatten their cities. And destroy just about anything of value elseware until they surrendered in full. Cost is irrelevent at that point. The nation will rage for revenge.
As for rebuilding. Something tells me Iran or whoever would have to become the next state before we would rebuild a country that hit us with a chem attack.
But using a nuke invites a war that humanity wont win. Are we going to have a press conference saying we launch in 3 days or whenever the wind is right? Maybe it needs to go to a commitee to determine long term effects? UN summit? About the only way you can be halfway confident nothing terrible will happen but by that time the cities would be looted and abandoned anyway.
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