At the start of Barack Obama’s term, few moments better crystallised America’s change of face than his “New Day” video address to the Iranian people. Ending with a salutation in Farsi, Mr Obama offered Iran a new era of “co-operation”. It followed from the promise in his inaugural address to “extend a hand if you are willing to unclench your fist”.

Unhappily, Tehran has ignored Mr Obama’s overtures. Not only is Iran thought to be within striking distance of how Israel defines its “zone of immunity” – the point at which Iranian nuclear weapons capability is irreversible – it has also threatened to block the Strait of Hormuz. Nothing spells global recession faster than $200-a-barrel oil prices. And nothing will bring a colder sweat to Mr Obama’s electoral strategists than the thought of conflict with Iran.

Yet that is what many in Washington are now calling a “probability”. Some White House officials have referred to 2012 as the “year of Iran”. It was supposed to be dominated by the US economy. Might it instead be the moment Mr Obama takes that 3am phone call?