While the Obama administration hasn't ruled out direct US military action against Iran, the United States would rather focus its energies on the overflowing plate of challenges it already faces, at home and abroad. And the American electorate has little enthusiasm for another war after almost nine years in Afghanistan and more than seven years in Iraq.
A threat deemed unacceptable
But it's almost impossible to imagine Jerusalem accepting a nuclear-armed Iran.
If tougher economic sanctions aren't seen very soon to be doing the job, then military force seems likely. Israel's Likud government, with support from other political parties, has publicly declared a nuclear Iran to be an intolerable existential threat.
And since its creation, Israel has demonstrated an inclination to follow up on its warnings to its enemies even if its own collateral costs are severe.
The gravity of the threat that the Israelis perceive in a nuclear Iran – particularly one with Mahmoud Ahmadinejad as president – means that simply forbidding a military strike is not one of the Obama administration's options.
Washington may have had the power to dictate to Israel (as well as Britain and France) to end the Suez crisis some five decades ago, but the evolution of domestic politics since Eisenhower's days has diminished US leverage over Israel. And Israel in 1956 was a poor third-world country, whereas today it is a self-confident, wealthy, high-tech economy.
Even if it can't forbid an Israeli strike, the US could publicly dissent. But if the Israelis were to attack Iran despite Washington's express objection, the closeness of the bilateral relationship would render the US vulnerable to the same blowback as if it had been an enthusiastic backer.