Mary Dejevsky: Obama is right. Britain, too, must seize the chance to leave Afghanistan

The announcement of the accelerated American withdrawal highlights the extent to which Britain has been left chasing shifts in US foreign policy

A "responsible end" is about as elegant a formulation for the victory so conspicuously not won in Afghanistan as President Obama could have found. And it is not only military victory that remains elusive, but many of the higher purposes that have studded this near decade-long campaign – women's rights among them. Obama, to his credit, had progressively returned to the mission's original justification – rooting out the al-Qa'ida bases held to harbour the instigators of 9/11 – but he was immeasurably helped when he succeeded where George Bush had failed: in tracking down and killing Osama bin Laden.

In the minds of Americans, and not just Americans, the elimination of Bin Laden was the single reason why US forces entered Afghanistan at all. Essentially, the whole campaign was a long and often frustrating reprisal raid for the destruction of the World Trade towers, the assault on the Pentagon and the loss of United Airlines flight 93. Of course, Obama was fortunate in that Bin Laden was cornered on his watch; but he made his own luck by weighing the odds and risking the order. His reward is the chance to reverse the 18-month old "surge" – which he authorised against his better judgement – and start an accelerated withdrawal of US troops.

The significance of this moment cannot be over-stated. The death of Bin Laden produced a constellation of circumstances that is unlikely to be repeated: the mood of America which regards a chapter as ended; Obama's own ascendancy over his top brass; the weakness, but not yet collapse, of the Karzai regime in Kabul; the growing power, but not yet domination, of the Afghan Taliban, and Pakistan's embarrassment that Bin Laden was found within its borders. Together, these offered Obama a now-or-never opportunity, and one he has seized with that air of low-key alacrity he has made his own.

Here in London, the scene could not have been more different. The US and British experience of Afghanistan has furnished a salutary study in comparative power, and the 24 hours before Obama announced his "drawdown" found Britain scrambling, as so often, to catch up. Thus the Foreign Secretary, William Hague, suddenly turned up in Kabul; we also learnt that the British had been conducting their own talks with the Taliban – a week after confirmation from President Karzai and the Americans that they had, as long rumoured, begun talks of their own. And now the Ministry of Defence is broaching the possibility that British forces – of whom a mere 400 (of 9,500) are due to be withdrawn this year – could return more quickly.

If they do not, and the US keeps to the timetable Obama has outlined, there is the theoretical prospect of British troops outstaying the Americans. That will not happen, but it is beyond doubt that the US is making all the running. No number of conversations between Obama and David Cameron – such as the one prominently reported in Britain in the hours before Obama's address – will change this.

The announcement of the accelerated US withdrawal from Afghanistan highlights the extent to which Britain – which still fancies itself to be US ally Number One – has been left chasing shifts in US foreign policy. The most awkward instance of British lagging behind came shortly after Obama had taken office, when ministers found themselves sharing platforms with their US counterparts at the Munich Security Conference and producing "Bush-speak" on Afghanistan and Iraq, even as US officials, who included the late Richard Holbrooke, were taking the new, and less belligerent, Obama line. In the months that followed British foreign policy was left in limbo, as Obama reviewed Afghan policy and delayed his decision.

The necessity to wait around for directions from across the Atlantic, however, has not been the only unwelcome message to be conveyed to Britain by the experience of Afghanistan. This long drawn-out campaign has brought home two other, equally unwelcome, realities of 21st century British life as a medium-sized power.

One is the gap shown to have opened up between US military capabilities and ours. At the outset, it was the digital and electronic capacity of US operations that impressed; then the superior quality of body-armour and armoured transport. The final humiliation was the failure – though it was not presented as such – of the effort in Helmand, where British troops were replaced by a bigger, and harder, contingent of Americans. The British had long contrasted their derring-do with the risk-aversion of the Germans, supposedly relaxing on their sun-loungers in Herat. The US saw less of a distinction.

The other is the extent to which two British prime ministers appear to defer to the top brass, to the point where the constitutional primacy of an elected government over the military could have been compromised. The memoirs of some senior diplomats, including our former man in Kabul, Sir Sherard Cowper-Coles, may enshrine their own institutional perspective, laced with elements of self-justification. But the way in which, as they tell it, the top brass held the politicians over a barrel, on budgets, manning levels and equipment, speaks of the chasm that now exists between the military and civilian worlds, and the formidable lobbying power senior commanders can wield. "Use them or lose them" is no justification for volunteering troops needlessly for other people's operations.

In scornfully dismissing the latest special pleading from commanders over operations in Libya – admitting that there were times when he felt like saying "You do the fighting, I'll do the talking" – Cameron shows that his government may be starting to redress the balance. But Afghanistan will only have the instructive effect it should, if the other experiences – the limits of British military power and the policy liabilities of being a very junior ally – are absorbed, too.

In the meantime, the unique combination of circumstances that permitted Obama to announce a "responsible end" to operations in Afghanistan also applies to Britain's presence there. As was repeated ad nauseum about alliance operations in Kosovo, "We went in together, we'll leave together, too" – with one footnote: the sooner this departure, the better.

m.dejevsky@independent.co.uk