If the covert option failed, the US would be informed before 2012, after 2012 if the covert option failed or the military option was chosen, then they may or may not. If sanctions are imposed or aid cut to Israel, it must happen after the operation not before to prevent it. The US knows of the Israel plans to a degree and that Israel has been planning for the last 6 years. What do people think the merchants ships was about. The main threat to the US being drawn into a war via an Israeli strike was being in Iraq, that would place the US in the situation in which they would have to bomb Iran, as was the case in 2007 and Panetta threaten Iran recently. Iran would be operating covertly as they did prior in Iraq. Israel should not draw the US into a war, Iran will have to act overtly attacking US bases, it is less likely, than covert action in Iraq. If they act overtly then they know the US would have to attack and they will be bombed into the stone age, it is suicide. Iran has never confronted the US directly it is always covert via proxies. Israel knows that the US controls countries via the military, that is why the command was replaced and prior too access at times was denied to the US. It has to occur before Putin walks into the Kremlin and while Syria is in chaos.
Protests in southern Syrian city after Druze cleric's death (AP)