Barack Obama's foreign policy team knew that sooner or later they would face a crisis over Iran. Unfortunately for the new US president, the crisis is already upon them.

On Friday, the Financial Times reported that “Iran has built up a stockpile of enough enriched uranium for one nuclear bomb”. That same day, Benjamin Netanyahu was invited to form Israel's next government.

Mr Netanyahu thinks that the Iranian government is “preparing another Holocaust for the Jewish state”. He has said: “It is 1938 and Iran is Germany.” Mr Netanyahu said this in 2006, so logically it is now 1941 – but the intervening years have not calmed him down. He thinks that an Iranian nuclear weapon would be a mortal threat to Israel.

As for President Obama, he has promised to “do everything in my power” to prevent Iran getting nuclear weapons. But how can he stop them?

A huge clue to the administration's approach was given by Gary Samore in a speech in Israel on December 18. Shortly afterwards, it emerged that Mr Samore would handle the non-proliferation job in the Obama White House. His words carry weight.

Mr Samore said there was “a growing sense in the region and more broadly that Iran's nuclear effort is unstoppable”. But he gave three reasons why an Obama diplomatic initiative might just work. The most important is the collapse in world oil prices, which makes Iran more vulnerable to economic sanctions. The second is that Mr Obama can make a credible offer of much better relations with the US. And third, Mr Obama's popularity overseas will make it easier for him to line up international support for sanctions.

But Mr Samore was far from sanguine that this would be enough. He noted that “Moscow and Beijing basically don't share our concern about Iran's nuclear programme”, and that Iran's leadership “probably value the acquisition of a nuclear weapons capability much more than better relations with the US”. He said: “We have to be realistic, and stopping Iran at this point is going to be a very difficult challenge.” The Iranians, he predicted, would attempt to “drag out negotiations . . . while they continue to build up their enrichment capability”. So the US should set a deadline for the suspension of enrichment. The initial deal should trade suspension of enrichment for suspension of sanctions.

The Obama administration will do its utmost. But if the diplomatic effort fails, Mr Samore thinks that the US will be left with “two unappetising choices of either trying to manage Iran with a bomb or bombing Iran”.

Iran's nuclear progress means that the US may have to face this “unappetising choice” rather sooner than the Obama team anticipated. The latest news from Iran suggests that the Iranians could produce material for a single bomb within months – although that would require a very public and detectable reconfiguration of their nuclear facilities.

So what should the Americans do? Mr Netanyahu will tell them that there is only one choice – bomb. His argument is based on the assumption that the Iranian government is run by genocidal maniacs, who would actively welcome Armageddon. Given the religious rhetoric of the Iranian regime, this possibility cannot be entirely dismissed. But most strategic experts – even in Israel – do not think that Iran is bent on nuking Israel.

Ephraim Kam, of the Jaffe Centre for Strategic Studies at Tel Aviv University, is fairly typical in arguing that a combination of Israeli and US nuclear deterrence would mean that “Iran will not use nuclear weapons, not against us and not against any other country.”