Officials in key parts of the Obama administration are increasingly convinced that sanctions will not deter Tehran from pursuing its nuclear programme, and believe that the US will be left with no option but to launch an attack on Iran or watch Israel do so.But there is a strong current of opinion within the administration -- including in the Pentagon and the state department -- that believes sanctions are doomed to fail, and that their principal use now is in delaying Israeli military action, as well as reassuring Europe that an attack will only come after other means have been tested.
If Obama were to conclude that there is no choice but to attack Iran, he is unlikely to order it before the presidential election in November unless there is an urgent reason to do so. The question is whether the Israelis will hold back that long.
However, the Americans are uncertain as to whether Israel is serious about using force if sanctions fail or has ratcheted up threats primarily in order to pressure the US and Europeans in to stronger action.
American officials are resigned to the fact that the US will be seen in much of the world as a partner in any Israeli assault on Iran -- whether or not Washington approved of it. The administration will then have to decide whether to, in the parlance of the US military, "pile on", by using its much greater firepower to finish what Israel starts.
Feeding in to the considerations are the timing of the American election, including its bearing on Israeli thinking, as well as the pace of Iranian advances in their nuclear programme.
"It's not that the Israelis believe the Iranians are on the brink of a bomb. It's that the Israelis may fear that the Iranian programme is on the brink of becoming out of reach of an Israeli military strike, which means it creates a 'now-or-never' moment," he said.
"That's what's actually driving the timeline by the middle of this year. But there's a countervailing factor that [Ehud] Barak has mentioned -- that they're not very close to making a decision and that they're also trying to ramp up concerns of an Israeli strike to drive the international community towards putting more pressure on the Iranians."
The presidential election is also a part of Israel's calculation, not least the fractious relationship between Obama and Netanyahu, who has little reason to do the US president any political favours and has good reason to prefer a Republican in the White House next year.
There is a school of thought -- a suspicion, even -- within the administration that Netanyahu might consider the height of the US election campaign the ideal time to attack Iran. With a hawkish Republican candidate ever ready to accuse him of weakness, Obama's room to pressure or oppose Netanyahu would be more limited than after the election.