The strategic timetable for the next nine months is becoming increasingly clear. Israel's apparent plans to strike Iran this year are limited by one crucial date: November 6 - the day of the US presidential elections.

Although he wishes with all his heart for a Republican victory, Prime Minister Benjamin Netanyahu is predicting the re-election of Barack Obama. A second-term president, not constrained by electoral necessities, will be able to apply a lot more pressure on the Israeli government not to attack.

Israel's window for action will probably close even earlier than November. The first reason for that is also electoral. Mr Netanyahu fears that a re-elected President Obama may find ways of supporting Israel's opposition parties, so he is expected to call early elections, probably by October.

The two men who are most in favour of a strike on Iran, the Prime Minister and his Defence Minister, Ehud Barak, will not wait for someone else to give the order - if an attack has to be launched, it is their destiny to make that call.

Aside from the American and Israeli election seasons, weather also affects the timing of a potential operation. While Israeli planners are certain that the air force has sufficient planes with the necessary range and payloads to cause enough damage to set the Iranian nuclear programme back, at least by three years, optimal conditions are needed to maximise that damage. That means a strike some time between May and September, when the Persian skies are clear of clouds.

The third factor is the Iranian effort to move its uranium enrichment process underground, to the subterranean installation near the city of Qom.

Mr Barak and other senior Israeli defence officials claimed last week at the Herzliya Conference that the Iranians are close to entering this "zone of immunity". The inference is clear: an attack will have to take place before that. If it does not, Israel will not be able to prevent a decision by Tehran to make a quick dash for nuclear-military capability.

Now that the timetable is clear, the terms of reference for the debate within Israel, and between Jerusalem and Washington, are also clarifying.

Messrs Netanyahu and Barak are convinced that action is needed before the centrifuges are moved underground and, if no one else acts, it rests on Israel to do the deed.

Some very senior figures in Israel's defence and intelligence community believe that this is not "the last chance" and that an attack at this junction will be counter-productive.

They are supported by the fact that the White House and Pentagon also believe that there is still enough time to let the new sanctions on Iran take effect before resorting to the military option.