Be careful what you wish for. Barack Obama wanted the American presidency, and with a brilliant campaign he won it. As late as early this summer, disbelief could still be suspended. Cartoonists were still depicting him as Superman, leaping over every problem mere mortals might put in his way. But he too has now been exposed as a mere mortal. He's not soaring over problems. Rather, he may be crushed by them.

"America – we are passing through a time of great trial," he told the assembled cadets at West Point as he presented his new strategy for Afghanistan, and never was a truer word spoken. His country is struggling to cope with unprecedented deficits and debt, stretching as far as the eye can see. Iran grows more truculent by the day. His signature issue of healthcare could well die a slow death in the Senate. We knew the chalice he inherited was poisoned. We just didn't realise quite how venomous was the brew inside. And then there's Afghanistan.

Watching Obama make the most important speech of his presidency on Tuesday was a profoundly depressing experience – and not just because the man who will receive the Nobel Peace Prize next week is plunging deeper into war.

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Yes, the address was deftly calibrated. The generals have got most of what they want; if Nato chips in as promised, General Stanley McChrystal's request for 40,000 men will roughly be met. Liberals, and not only liberals, in his own party can be consoled at the prospect of a withdrawal starting in mid-2011, just when Obama's re-election campaign will get going in earnest.

But it's virtually impossible to believe this timetable. The goal is to "disrupt, dismantle and defeat" al-Q'aida, and to break the momentum of the Taliban. That however is a huge gamble, however intense the pressure on the Karzai government and however skilful the suborning of tribal leaders. Can either the central or local Afghan authorities really start taking over from the US within 18 months, even if the extra troops put the Taliban on the back foot?

And then there's the even huger gamble of Pakistan, without which a durable success in Afghanistan is impossible – but whose size, nuclear weaponry and instability make it even less susceptible to American influence.

Yet he failed to convince that this rolling of the dice was worth it. Watching and listening to Obama, you felt this was a commander-in-chief whose heart (and stomach) were not in this war, but whose head, guided by political realities, had convinced him he must continue. You can't blame him for that: politicians who ignore political realities have a very short shelf-life. But in a deeper sense, that's why the spectacle was so sad.

Right-wing commentators in Washington have been praising Obama for his "courage" in deciding to escalate an unpopular war. They point out how in late 2006, when his presidency was at a far lower ebb than Obama's is now, President George W Bush ordered his surge in Iraq, in the teeth of fierce opposition, including from within his own party. And it is true that the surge has at minimum allowed the US to start the withdrawal now under way (though whether it has brought real stability to Iraq is quite another matter).

That is the most Obama can hope for in Afghanistan. But real courage this time would have been to declare, if not a withdrawal, at least that no more troops would be sent. Despite his despatch of more soldiers earlier this year, a window was still open to reverse course. Afghanistan until Tuesday night was still Bush's war. No longer.

The logic of the address didn't add up. Everyone knows that Afghanistan was a punitive war, to eradicate the al-Q'aida presence from the country after 9/11 and capture Osama bin Laden and his high command. But that last goal failed, and though al-Qa'ida seems to have been weakened there, the virus it carries has spread much further afield. If Afghanistan, then why not Somalia, Yemen, or somewhere else?

A direct no to the generals, or a massive boost in troops, would both have made more sense in military terms. Instead, Obama presented a plan that split the difference. The immediate reaction has been relatively favourable. Remarkably, according to a new Gallup poll, 51 per cent of Americans support the new strategy. But that surely reflects the lingering popularity of Obama the man, rather than any new embrace of a war of which the country is heartily fed up, and about whose outcome it remains pessimistic.

And this fragile approval may well evaporate when the Afghan war season returns in the spring, when a larger American force starts to sustain larger casualties. What then? What happens if this surge produces no measurable results? Come 2011, it is far more likely he'll be announcing a delay in withdrawal's start, or even be facing demands from his generals for even more men "to finish the job."

More than 40 years ago, an earlier Democratic president named Lyndon B Johnson, with a no less ambitious domestic agenda than Obama, was confronted by the same choice. We all know how Vietnam ended. No two conflicts are exactly alike. But with every passing month, the similarities between Vietnam and Afghanistan grow.

George Orwell once observed that the quickest way to end a war is to lose it – but losing a war is also the quickest way to lose a presidency, too. That reasoning prevailed with LBJ, though Vietnam forced him out of the White House regardless. For Obama, the stakes are as high now.

An open-ended commitment to nation-building in Afghanistan was out, he said at West Point, "because the nation that I am most interested in building is our own." Obama's quest is for a fairer society, offering health care for all and an end to financial excesses,, and an America whose future has not been hopelessly mortgaged to pay for the feckless present.

The great promise of this presidency was that it would usher in a new way of doing politics. It would, as they say, break the mould. But the Afghan speech, so full of politics as usual, gave the lie to that. We all should be careful what we wish for.