Unless Barack Obama has a sudden change of heart, his address to the nation will privately disappoint many of the senior commanders in Isaf – the International Security and Assistance Force – and provide them with immediate logistical problems.

The military had been expecting an initial withdrawal of between 3,000 and 5,000 troops. That can be achieved relatively easily by reducing the number of back-up staff in Afghanistan.

The ratio of fighters to support personnel is out of kilter at the moment, so this would help to rationalise the "teeth to tail" numbers. A further 5,000 soldiers out by the end of the year is more of a problem.

In military circles this target is described as "challenging". That means it is going to be a real headache, potentially disrupting plans for this summer's fighting season, and the strategy for the autumn.

Speaking ahead of the speech, Professor Michael Clarke, director of the Royal United Services Institute thinktank, warned the US needed to keep "as many combat units intact for as long as possible" or risk "snatching defeat from the jaws of victory".

But the likelihood now is that the US will have to start withdrawing one of its brigades in the autumn, to meet the first end of year deadline, and then synchronise the withdrawal of others through 2012 to get all 33,000 surge troops out of Afghanistan within the White House timetable.

There will not be a second fighting season at full strength, which is what commanders wanted.

All talk of a "conditions-based withdrawal", the phrase used by Downing Street and the White House to provide reassurance to the generals, appears to have been abandoned. The conditions driving this process now are political ones.

In early 2013, the US will still have more than 60,000 troops in Afghanistan. And unless David Cameron changes his mind, there will also be 8,500 to 9,000 British troops in the country – the prime minister is expected to sign off plans to withdraw 500 troops next year, in addition to the 426 that will come home this summer.

But by that stage, the long march out of Afghanistan will have begun in earnest. By the end of following year the bulk of US and British troops who are at the sharp end of the fighting will have returned to base.

Whitehall officials expect about 2,000 British troops to remain in Afghanistan in an overwatch and training role. There will be more Americans than that, but they will be taking a back seat, too.

The onus will be on the Afghan national army, and Afghan police force, to take responsibility for the security of a country that is riddled with corruption and still in hock to warlords. It is a big ask.

Willing as the recruits may be, most of those who join have to undergo basic literacy training first – a low starting point that initially had the US General William Caldwell, who is in overall charge of training, burying his head in his hands.

They will be shouldering responsibility for taking the fight to insurgents who, in all probability, will have outlasted the Nato military effort.

Obama threw the dice when he ordered the surge in December 2009 to pursue the "war of necessity". He is gambling again – hoping the Afghans are strong enough to take control, that the Taliban may be brought to the negotiating table in the meantime, and that there will be a "withdrawal dividend" that will give President Karzai's successor a chance to unite the disparate tribes of country.