My biggest fear is that Israel goes-it-alone and then Iran, stupidly, decides to retaliate at least in part against the US. I think that's the mostly likely scenario at this point.

It seems pretty obvious to me the President isn't interested in a preventative war (thanks, Mike, for the definition for that one), the intel community still says the Iranians haven't restarted weaponization work, I doubt the US will attack absent some fundamental change on the ground.