My guess is that the US, failing to build a Coalition of Willing to say nothing of UN consensus, will have Israel execute a bombing raid on Iran. The Iranian military response will be limited to what it can shoot down during the attack (if anything). The result will be the galvanizing of anti-US sentiment in Iran and a propping up of the current Iranian leadership. China and Russia will assume the role of Iranian supporters spearheading international condemnation and aiding domestic recovery. This will move Iran inexorably further adrift of US influence and further into the Central Asian sphere (i.e. Shanghai Cooperation Organization security club). Israel, beyond defeating an essentially non-existent threat, will do little more than damage its already dismal international reputation. Such would mean little to the leadership of course but it could have the undesirable effect of adding impetus to Boycott Israel efforts. This assumes that Iranian targets will be limited to strategic nuclear infrastructure. If on the other hand a wide ranging attack were executed, one that targeted significant military and/or civilian targets all bets would be off as Iran would then retaliate (immediately) to the greatest extent possible. If the Iranian capacity to inflict damage is substantial then major disruptions of the global economy would follow. The utility of having Israel conduct the attack may them be beneficial to the US should world pressure demand a pound of flesh in retribution.
U.S. defense chief tells Congress: Military options remain against Iran (DPA)
from the article: Preparing for the military option