If Iran wasn't deterred by 200 or so Israeli nuclear weapons pointed at them from a few hundred miles, the prospect of the USA chipping in as well isn't too likely to change things.

The reason Iran might seek to develop nuclear weapons would be to deter attacks, such as the war of aggression launched by Iraq a few years back under Saddam when he was a valued US ally, or an invasion comparable to that carried out against Iraq five years ago, when Saddam had ceased to be a valued US ally, or an attack by Israel, with its massive nuclear arsenal, and US backing.

Those are real and reasonable fears, and they need to be addressed by firm guarantees that no such attacks will be waged or tolerated, even if carried out by friends of the US.