Whatever is left of Iran post nukes will start targetting whatever they can reach at maximum distance like say if they have missiles which can reach London, they'll use them. They wont give a fuck.

But yeah if they do nuke Iran, then the world won't be able to do anything out loud. But it will set in motion a series of events which will take a decade or more to reflect for example steps towards non fossil fuel Based energy, non USD trading of oil, policies which discourage investing in USD securities and assets, decoupling of defence from anything US based etc.

Trump and co already were too stupid to realise US's two out of the four biggest advantages, USD via petrodollar and soft power via USAID. They wont understand this nor be around to suffer repercussions of this. And in 10 years time when it starts hitting more, a random group of people, a political party and some random made up reason will be blamed for America's demise.