There’s a great scene in Thirteen Days, the film about the Cuban missile crisis, in which Kevin Conway as General Curtis LeMay tells President Kennedy: “The big red dog is diggin’ in our backyard, and we are justified in shooting him!”

Kennedy takes a deep breath and a walk along the West Wing colonnade, and rejects LeMay’s advice.

If Barack Obama has seen the film, he will not have forgotten that scene. His generals are not urging him to rain nuclear bombs on the Taleban, but they are putting him in the desperately lonely position that only a President can know, in which as Commander-in-Chief he must give orders to military men who know vastly more about military affairs than he does.

General Stanley McChrystal has decided that to fulfil his mission – to disrupt, dismantle and defeat al-Qaeda in Pakistan and Afghanistan – he needs more soldiers and a new strategy.

That mission was formally defined by President Obama in March. It was not a surprise then; he campaigned on the idea of the Afghan conflict as a war of necessity and has talked with something close to relish about killing Osama bin Laden. Nor has the mission changed much since the spring.

Asked about Afghanistan in five slightly different ways on Friday last week, the President answered clearly that the point of fighting there was to hurt al-Qaeda so badly that it could not threaten the US from Central Asia again. Or, as it put in on The Late Show with David Letterman last night: “My central objective is that we take those folks out.”

His man on the ground has told him how the task can be accomplished. His top uniformed adviser in Washington, the Chairman of the Joint Chiefs of Staff, has publicly urged him to give General McChrystal everything he wants. And yet the President hesitates.

Mr Obama has taken more than a walk round the West Wing. He has postponed a decision on a new Afghan surge indefinitely – probably until November at the earliest. His staff are even reported to have told General McChrystal not to bother sending any specific troop requests until they tell him that the time is right.

Why? Because so much has changed since March. The insurgents have, by the General’s own admission, wrested the military initiative from Nato. The 21,000 US reinforcements already authorised by Mr Obama delivered reasonable security for the elections, but they could not prevent August becoming the worst month in eight years for allied casualties.

They could not quell mounting domestic impatience with a war that has already dragged on longer than major US military operations in Vietnam. And they could not prevent the wholesale ballot fraud that stripped President Karzai of what remained of his legitimacy.

Mr Obama can defend the defence of a corrupt regime, but only by arguing that there is no alternative from the point of view of US national security. His allies say there are alternatives aplenty.

Carl Levin, Democratic chairman of the Senate Armed Services Committee, believes the Afghan army is the solution and an increased US army presence would quickly become part of the problem. Joe Biden, the Vice-President, believes that the Afghan effort is already diverting resources from where they are really needed – in Pakistan.

Robert Gates, the Defence Secretary, says publicly that he is persuaded by General McChrystal’s arguments for more troops. Privately, he remains sceptical. So do senior White House officials who advise the President on security. One told The Times recently: “No one can put in enough troops for a comprehensive military win, but we’re not trying to build a 51st state of the union. Our goal is to deny al-Qaeda a base.”

He added: “There are enough troops, if co-ordinated with reconstruction and government, that we can be irreversible in terms of whether the population is going to swing to the Taleban or not.”

Over the debate hangs the shadow of Iraq. For Senator John McCain, the lesson of that war is to stay the course and back the generals when they call for a surge. For many in the White House it is that President Bush accepted uncritically the advice of his top military brass for four disastrous years before his legacy and America’s mission were saved from total ruin by General David Petraeus.

For every Washington pundit who claims Mr Obama has a secret timetable for withdrawal from Afghanistan, there is another who insists he is about to “double down” there. The truth is he seems genuinely undecided. General McChrystal and Senator McCain are right about one thing. He cannot stay undecided for long.

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