The British deployment to Afghanistan is set to reach 10,000
A surge of US military forces in Afghanistan is expected to result in better security for Helmand Province, where the bulk of UK forces are based.
The US deployment and 500 extra British soldiers, is set to boost Nato's troop numbers in the most crucial areas.
It should also allow them to accelerate the training of the Afghan Army and police, now a key focus of the mission.
Senior British military sources say that some areas could be under Afghan forces in 2010.
They say there is no fixed timetable, but the hope is that some districts in Helmand could be sufficiently stable to be handed to Afghan forces over next year, although that will depend on how well they perform.
Military commanders remain wary of talk of exit strategies or withdrawal dates, preferring those decisions to be based on conditions on the ground.
Exactly where the extra US Marine forces will go in Helmand is not yet clear, but they are likely to focus on the areas that remain Taliban strongholds.
The Americans could also reinforce areas further to the north of the province, in areas where British troops are most thinly-spread.
Currently, the US force in Helmand of some 12,000 US Marines is focussed in the south of the province, with British forces mainly in the "green zone", the fertile area around the Helmand river, and further to the north in Sangin, Musa Qala and Kajaki.
Crucially, President Obama's announcement sends as much of a political as a military signal
For the US reinforcements in Helmand, the first priority is likely to be to dislodge the Taliban from the town of Marjeh, which remains a hub of the country's opium trade.
It is also a Taliban stronghold where many of the roadside bombs are made. They remain one of the biggest dangers facing western troops.
At the centre of the US Nato commander Stanley McChrystal's vision is a focus on a credible strategy which will ultimately be sustainable by the Afghans themselves.
That will mean growing Afghan forces - both the Army and police - considerably faster than has been achieved until now, with an equal focus on governance.
Some of Afghans' biggest grievances involve a lack of access to justice - crucial in a society ravaged by war for three decades, and one in which blood feuds are ingrained in the culture.
The Taliban has succeeded in some areas by offering the rapid resolution of feuds and disputes over land or livestock.
Its ability to form "shadow governments", which may be harsher but in many ways more effective than the actual government, has enabled their influence to spread.
So a political push will be just as crucial to help build up governance.
The key tasks will be to protect Afghans in the most populated areas, to set up local economic and social programmes, and encourage "reconcilable" Taliban fighters to come over to the coalition.
According to Gordon Brown, when the Afghan National Army is built up from its current level of 90,000 to 134,000, the British government could review the future for British troops.
Mr Brown confirms conditions for extra troops to be sent have been met
"It is at that point we could look at what is the need for British troops if the Afghanistan people are able to be responsible for their own security, but not until then," he said recently.
Crucially, President Obama's announcement sends as much of a political as a military signal: That the US, Britain and - they hope - their other Nato allies are absolutely committed to one last push to make it work in Afghanistan.
But this is all part of a difficult balancing act. For their domestic electorates, politicians in America and the UK are emphasising that this is the beginning of the end, and that their nation's stretched Armed Forces will soon start to come home - even as more are deployed in the short-term.
Yet at the same time, the Afghan people - who are at the heart of the counter-insurgency strategy - must be persuaded by President Obama and others that the West will stay as long as is needed, at least until Afghan security forces are ready to take over.
The Taliban - who at the moment have every reason to believe they can outlast western forces - must also be persuaded that they are not in the ascendant, otherwise there is no reason for lower or middle-ranking supporters to switch sides and join the Afghan government and the coalition's efforts.
Some remain sceptical about whether the Afghan army and police can be trained to take over as quickly as hoped.
The Afghan army loses up to a quarter of its recruits, many of whom simply walk away, and has struggled to generate the numbers required by the plan, while the Afghan police are still widely mistrusted.
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