By Gordon Corera
BBC security correspondent
The UK public are questioning whether the mission can succeed
One hundred UK service personnel have now been killed in Afghanistan this year alone, after a soldier from the 1st Batallion the Royal Anglian Regiment became the latest casualty on 7 December.
The casualty toll in Afghanistan has been increasing steadily from 2006 when British troops were first deployed to Helmand province.
But there has been a sense in recent months that the mission has reached a critical point - not so much in terms of its successes or failure on the ground, but in the debates back home amongst the public and politicians.
The toll has been rising largely thanks to the Taliban's use of improvised explosive devices - 80% of British deaths this year have come from these makeshift bombs.
But individual incidents, like the killing of five servicemen by a rogue Afghan policeman in early November or the period in July when eight British soldiers were killed in 24 hours, have had a significant impact on the debate over whether troops should stay in Afghanistan, and how successful the mission has been.
A spike in the number of casualties occurred in the summer with Operation Panther's Claw.
It was, in part, undertaken to secure territory ahead of the Afghan presidential election.
But the accusations of fraud surrounding the poll and the low turnout - including in areas covered by Panther's Claw - also intensified the debate over the value of the mission.
On 17 November 2009, Foreign Secretary David Miliband said Afghanistan would not be a "war without end".
On the previous day in his Lord Mayor's Banquet Speech, the prime minister spoke of establishing a process for eventual withdrawal based on handing over security to Afghan forces.
This process could start soon in districts where Taliban influence is weak. But in other areas - especially in Helmand where British troops are operating - such a transfer could be years away.
Training and building up Afghan forces remains central to the strategy.
Five UK soldiers died at the hands of an Afghan policeman in November
"From a military perspective, our objective is to get the Afghan national security forces into the lead in this counter-insurgency operation," Britain's chief of the defence staff, Air Chief Marshall Sir Jock Stirrup told the BBC on 2 December.
The coalition needs an effective partner on the Afghan side, but this has been hampered by the disarray over this year's election which has been perceived as a challenge to President Hamid Karzai's legitimacy.
An effective partner on the Afghan side is crucial and the disarray over this year's election has complicated that process and challenged Hamid Karzai's legitimacy. The US appears to have decided that while he may be an imperfect partner, Washington has no alternative.
The Taliban would need to be weakened to allow Afghan security forces to tackle them.
US General Stanley McChrystal argued that to accomplish this, more troops would be required in the short term in order to change the dynamics of the conflict against the Taliban.
Paradoxically, it is argued that only by putting in more troops will it be possible to then prepare for their withdrawal.
Critics in Washington say that the argument that more troops are needed was exactly that used in Vietnam and led to a long escalation but not before it was understood the conflict was not winnable in traditional terms.
The UK is also hoping that it can weaken the insurgency through what it calls "reconciliation and reintegration" - persuading Taliban to leave the fight and drawing them into the political process.
This is only likely to happen if the Taliban stop believing that they are winning and can simply wait for what they see as the inevitable withdrawal of foreign troops.
It is a war very much worth fighting, for our sakes and our childrens' sakes, and the price of failure hasn't been understood
General Sir David Richards
Head of the British army
The three month wait for President Obama to announce his new strategy heightened uncertainty.
With the announcement of an influx of more than 30,000 US troops on 1 December, the president made clear that he would increase the US military commitment to Afghanistan in the short term, but also aim to start some kind of withdrawal in mid-2011.
The talk of withdrawal was seen as vital in selling the build-up to a sceptical American public and Congress. In the short term, more troops will mean more fighting and more casualties.
The rising casualty toll has certainly changed perceptions of the conflict both in the US and UK. In the UK, opinion polls show strong support for the troops but growing uncertainty over the mission.
The government has been emphasising that British troops are fighting in Afghanistan to keep the streets of Britain safe.
Their argument is that ceding control to the Taliban would allow al-Qaeda to once again establish training camps and plan attacks from Afghanistan.
Preventing terrorism was the original motivation for intervention in 2001.
The argument is indirect however, since UK forces are not fighting al-Qaeda itself - and some analysts question whether al-Qaeda and the Taliban would resume their pre-9/11 relationship and whether the Taliban are really likely to be defeated.
But within the military leadership, the belief that the mission is vital remains strong.
"[The mission] is absolutely worth doing," Gen Sir David Richards told the BBC in October.
"Every one of our soldiers out there believes that instinctively and in the way he practices every day. It is a war very much worth fighting, for our sakes and our children's sakes, and the price of failure hasn't been understood," he said.
The government is keen to stress that this is not a war without end.
But no-one is yet predicting when that end will come or how many more casualties there will be.