It has been/and currently is US policy that WMD=WMD. That is, if you use ANY form of WMD against we will respond with nukes - being as the US has no chem or bio weapons.
Baghdad is slagged, and a few small tac nukes are used on suspected/real WMD sites & major military bases. The Kurds form Kurdistan - a US protectorate, and Southern Iraq, where most of the oil that is not in Kurdistan is is occupied by the USA with significant bases erected and oil money taken in perpetuity as reparations. The local population is given the choice of conforming to US standards (with freedom of religion), English becoming the language of government & commerce etc or leaving. Any attempts at insurrection are brutally repressed, and families of convicted insurrectionists are expelled.
Needless to say the Iranians are not so subtly informed that any misbehavior on their part, whether local or via proxies such as Hizbollah & Hamas will result in nuclear urban renewal in Tehran and Qom.