There are many scenarios where stupidity could break out between Iran and Israel that would not lead to a full blown exchange that would bring an end to the oil supplies in the Middle East. Most of the rest of the world would be very reluctant to jump in with both feet if those two started throwing nukes at each others. And, unless either decided to throw to rogue targets, the damage would be limited to those two countries.
It would suck to be in the region and the world's economy would shutter hard but it would stabilize once the two combatants were put in their corners. Both are small enough that they can't take many nuclear hits without capitulation. Then the maps change again.
I think both of these countries are loony. Neither has anything to offer the rest of the world but at least Iran has oil. I would like to see nothing done and Iran get the nukes. What can happen if Iran does get nukes? 2 things:
1. Peace in the region because Israel would be forced to behave due to another and pretty much equal power in the region. Israel only behaves like a bully in the region because of its power. This needs to be stabilized.
nuclear war. Both countries would be destroyed. Start over with a clean State.
However, I do not believe that the United States should be involved.