It is not sure the US will stay Israel's warlords from a preemptive air strike on Iran. The US may not steer the Likud government towards such action, but there is good reason if past history is any signpost, to posit that Washington won't impose it if US president Obama [BHO] and his EU allies and Russia cannot bring Iran around to some mutually agreeable solution to Tehran's nuclear programme. And even then, there is no guarantee that loose cannon Israel will do it anyhow, this inspite of doubts and objections to hawkish Israeli designs.
Of course BHO is no George W. Bush who formally shot down Israel's petition of the US green light to bomb Iran in 2008. But BHO is falling back on and pushing the envelope more on a return to a harder line on Teheran.
Were Netenyahu impetuous enough to allow such action, the US for sure will immediate disassociate itself from such an operation only after damage has been done.
The Israeli defence ministry and government are banging more often the pots and pans for preemptive strikes. Some will say, this is bluffing, others will disagree, saying, it's a matter of months before such attacks will occur.
If they do, Washington will have to back peddle so hard and so fast since an Israeli strike on Iran will lose BHO any credibility in the Arab world, cause war with Israel by the Arabs, cut off oil & gas supplies to the US, and put Washington in a war it may not have wanted but courted by not keeping Israel in check.