The decision by Abdullah Abdullah to pull out of the run-off in the presidential election in Afghanistan should come as cold comfort to the incumbent, Hamid Karzai, whose claim to legitimacy as head of state, never wholly accepted abroad or at home, looks weaker than ever. As for Western strategists, particularly those in the US and Britain – the two countries that supply most foreign troops to Afghanistan – this is the nightmare scenario.

Barack Obama now faces an appalling dilemma as he considers the request of his commander on the ground, General Stanley McChrystal, for another 40,000 troops to prop up the Kabul government's fight against the Taliban. Having insisted that Washington would only continue to support a government whose democratic mandate was beyond reasonable question, the US President is hostage to that pledge, damned if he sends more troops and damned if he doesn't.

Parallels between the South Vietnam and Afghan governments, and between the Taliban and Vietcong, once derided as far-fetched, now look closer. Mr Obama must surely be haunted by the ghost of Lyndon B Johnson – another reforming Democrat president whose dreams of transforming American society fell victim to its creeping involvement in an unpopular, unwinnable war, the goals of which were never really established.

The irony of the latest developments is that the Western allies of the Afghan government did not particularly object to Dr Abdullah's withdrawal from this weekend's planned presidential run-off. Of course, the politicians said no such thing in public, honouring the fiction that Dr Abdullah and Mr Karzai needed to submit their claims to the electorate once again in an election that was less obviously fraudulent than the first round.

But the worry was always there that the Taliban would use another round of voting to unleash a spectacular orgy of violence in order to demonstrate their growing strength in the country. At the very least, the fear was that they would intimidate so many people into not voting as to render the re-run a farce.

The discreetly maintained hope in Washington and London was that a last-minute power-sharing agreement, saving everyone's faces, would render a run-off unnecessary. This might have involved Mr Karzai remaining president, Dr Abdullah being drawn into the fold in a significant position and a more broadly based government uniting to prosecute the war against Islamist extremists more efficiently than before, and with the support of an expanded US military presence.

Now all bets are off and the possibility arises of Western troops having to shoulder more and more of the burden of the conflict – a development that would only reinforce the Taliban's claim to be a nationalist resistance movement fighting foreign occupation.

It is possible that Dr Abdullah set the price of his involvement in a unity government too high and demanded too many concessions. Nevertheless, one can only wonder at the infinitely greater folly of Mr Karzai in spurning this opportunity to put his government on a more solid footing. Myopic to the end, he seems intent on burying his government's only long-term hope of political survival, not to mention our own hopes of eventually leaving his troubled country to a more secure future.