What if they "shoot one nuke" at Israel? Then what?
It seems to be general knowlege that Israel has "the bomb" so I would hope that America and Europe would stay out of the way and let Israel defend/avenge itself.
An exchange of two or three atom bombs in the Middle East will not destroy the planet - and would probably not even destroy Israel or Iran. The cost would be horrible for everybody and very likely nothing would be settled. It sure would insert discussion of nuclear non-proliferation and
disarmament into the news cycle (at least until the next sex scandal or baby-fallen-in-a-well drama...:dry

. This "happy" outcome assumes that when one or two bombs start flying the atomic superpowers stay out of it.
Between Israel vs. Iran, India vs. Pakistan, and North Korea vs. everybody I really expect such a small-scale exchange of half-a-dozen bombs to take place in my lifetime (and I will soon be 52). I don't see any way for the us to stop it from happening. The best outcome I can imagine is that it everyone else holds their fire, and that humanity generally is put off on using nuclear weapons for another 50 years.
What if they develop a nuclear weapon and then, some time later a nuke goes off in an American or European city, but the Iranians deny responsibility, saying that some group not connected to them did it? Than what?
Then we investigate, track down the source of the bomb as best we can, and obliterate whoever is responsible. We take it like men, pull ourselves together and respond with thoughtfull savy...
Or we could react as we did to 9/11, alternating between abject cowering, bullying swagger, and thoughtless flailing at every scary bogeyman on the planet. We could forego calm courage and abandon all our principles, in hopes of being completely safe. We could recall George W's stirring clarion call to action... we could all go shopping.
Before that happens we could take multiple measures: Increase surveillance of Iran (especially human intelligence), establish cautious relations with Iran (keep your enemies close - it has worked with Russia and China), and most of all disentangle ourselves from the region by eliminating our dependance on Middle Eastern oil. They don't "hate us for our freedom," they hate us because we are there (and because we support Israel). The "simple" solution is to get out. Ultimately, Iran has serious prospects for developing a bomb because they have lots of money, and that money comes from
us.
What we do not need is another half-baked "pre-emptive warfare" misadventure.