As the US military dramatically increases its numbers in Afghanistan there are real fears among British and US soldiers that they may have been given an impossible mission.
Coalition forces face a challenge to keep up with Obama's timetable
Our conversation had gone on amiably enough. The colonel and I sitting in a London cafe discussing current Nato operations in Afghanistan.
We sipped our drinks in the June sunlight and for an hour or so we discussed various operational issues; it had all been informative but hardly surprising.
Then towards the end his face darkened.
"Our main concern," he said, "is do we have enough time - both in Britain and the United States - to deliver some success, something we could use to satisfy the politicians."
The success they crave is a significant downturn in violence.
His coalition bosses share the worry about timings.
On Tuesday, during questioning at the Senate Armed Services Committee on the timeline issue, Gen David Petraeus fainted.
There is a gulf already between the White House and those running the war
The general, whose success in Iraq produces reverence on Capitol Hill, had already described his approval of the president's withdrawal timetable as "qualified".
From a man who chooses his words as carefully as Napoleon might have done his fighting ground, this sent a clear enough signal.
And as for the fainting, that would have been regarded by Roman generals as a bad omen.
So there is a gulf already between the White House and those running the war.
Saying one thing...?
Senior officers think more than trebling US forces from 31,000 in spring 2009 to 98,000 this coming August is a huge logistical task.
It will take time to deliver results in the struggle for the Afghan people.
Nato must win support even in places as troubled as Sangin
Nato must find partners, even in places like Sangin bazaar in Helmand where that battle is so complex and intense that an Alikozai tribesman fears murder if he shops with an Ishakzai trader.
And it is in Washington or London, far from those pungent alleyways, where the "ferenghi" (foreign) politicians will decide whether their soldiers can succeed or not.
Yet the Obama surge will only provide a window of maximum troop strength for just eight or nine months.
No wonder some officers use the phrase "mission impossible".
And as the numbers go up, so will Nato's casualties.
The death toll among coalition troops has risen each year in recent times.
Inevitably, the cries from those who oppose the strategy, particularly among President Obama's party base, intensify as the human cost mounts.
In Britain, many in the new government are also anxious to reduce national exposure.
Yet on Monday Liam Fox, the UK defence secretary, told an audience in London: "We are committed to seeing the mission through to resolution."
So are they saying one thing and plotting another?
Question of timing
There are two dimensions to the quandary the new UK Prime Minister, David Cameron, faces over Afghanistan.
The army is anxious to avoid "Basra 2".
In other words, if the US takes over places in Helmand province where the British army has been struggling to bring security, then, just like in southern Iraq, cigar-chomping American colonels may argue they have had to finish a job the Brits just could not.
This argument apparently won the day even with Sangin, where British troops suffer around half their casualties.
Britain could easily have palmed the district off on the Americans during a reorganisation of commands last month.
But the British army was against it.
It was in Sangin a few months ago that I had several long conversations with Lt-Col Nick Kitson, then commanding there.
Steeled by prior Special Forces service, Lt-Col Kitson is a mild-mannered enthusiast for the strategy of trying to win over the Afghan people.
Thirty men under his command had been killed in six months - but, he insisted, as we chatted by the stream that ran through his headquarters, they were beginning to turn the situation around.
Arguments like his have swayed the new government.
If there is any success to be declared in places like that, Britain wants to own it.
The other dimension of Mr Cameron's dilemma is political.
Sangin has a lethal reputation among coalition soldiers
Downing Street is nervous about drawing down before the Americans begin to cut their presence in earnest.
So everything comes back to timing, and President Obama's declared aspiration of starting to pull out in July next year.
Men like the colonel I had shared a coffee with in London, or indeed Gen Petraeus, know that stating things with a sense of urgency, as the White House has done, can communicate its own message of weakness.
Their fear is that this president could easily lose patience soon with the coalition's war and start to wind it down before the military have been able to get a grip of that wild country.
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