Binyamin Netanyahu, the prime minister, and Ehud Barak, the defence minister, will then have to decide if Israel can live with a nuclear Iran and rely on deterrence. But if they judge the risk of a nuclear assault on Israel too great, Israel's military will have to do what it can to destroy Iran's nuclear installations, despite the likely devastating repercussions – regional and global.

These will probably include massive rocketing of Israel's cities and military bases by the Iranians and Hezbollah (from Lebanon), and possibly by Hamas (from Gaza). This could trigger land wars in Lebanon and Gaza as well as a protracted long-range war with Iran. It could see terrorism by Iranian agents against Israeli (and Jewish) targets around the world; a steep increase in world oil prices, which will rebound politically against Israel; and Iranian action against American targets in Iraq, Afghanistan and the Gulf. More generally, Islamist terrorism against western targets could only grow.

But it is not only Israel's leaders who will have to decide. So will Obama, a man who has, in the international arena, shown a proclivity for indecision (except when it comes to Israeli settlements in the West Bank). Will he give the Israelis a green light (and perhaps some additional equipment they have been seeking to facilitate a strike) and a right-of-passage corridor over Iraq for their aircraft? Or will he acquiesce in putting atomic weaponry in the mullahs' hands?

It is clear – and should be by then to all but the most supine appeasers – that the diplomatic approach is going nowhere, with the Iranians conning and stonewalling and dragging their feet, all the while enriching more uranium. And Tehran is laughing, as it were, all the way to Armageddon. Ahmadinejad and the mullahs know full well that the west will never impose the only sanctions that could work (a complete boycott of Iranian oil and cessation of the export to Iran of all products).

Some in the west blithely hope that the Iranians are aiming for a low-key and shadowy "bomb in the basement", rather than immediately usable atomic bombs, and that this reduces the necessity of a pre-emptive military strike. My guess is that Iran has not taken this giant gamble in order to achieve a dubious, implicit capability: it will not stop short of actual, usable atomic weapons with which to overawe and gain hegemony over its neighbours, deter the west and, perhaps, destroy Israel.

So Obama is fast approaching his moment of truth. His predecessor, George Bush, repeatedly assured Israel that the US would not allow fundamentalist Iran to attain the bomb. The implication was that America itself would prevent this – at the last resort, by military means.

Today that seems highly unlikely. Obama is enmeshed in two wars in Muslim lands, with Afghanistan looking increasingly unwinnable, and Iraq stumbling either toward de facto partition or growing subordination to Shia Iran. With an American public increasingly tired of war, any war, the US president is unlikely to send in the air force, navy and special forces to smash the Iranian nuclear installations.

There is a sad double irony here. The Iranians and their proxies are likely to attack American targets whether or not the US is involved in a strike against Iran. And while Israel's conventional military capabilities are limited and could probably delay the Iranian acquisition of nuclear arms only by a few years, American conventional might – if brought resolutely and efficiently to bear – could completely halt Iran's nuclear project and thoroughly destroy its military carapace in a few weeks of intensive bombing; indeed, the regime itself might collapse like a house of cards, as did Saddam's under the American onslaught of March 2003.

This is not going to happen. Nevertheless Obama will soon have to decide whether to give Israel a green light, and how brightly it will shine. And soon. For spring is fast approaching.