Issues of UK's troop strategy change in Afghanistan
There is a military logic to the planned redeployment of forces in Sangin.
But there are also huge emotions and potential sensitivities attached to the withdrawal of British forces from what is generally acknowledged as an exceptionally difficult and dangerous area.
The military balance of power in southern Helmand has changed dramatically. The British, for so long the dominant military presence there, have seen a huge influx of US forces, part of President Barack Obama's surge strategy. US forces now predominate in northern Helmand - where Sangin is - and the south.
Overall, there are now about 20,000 US troops in Helmand, compared to around 8-9,000 British troops.
There is now an American general in charge in the province, after a shake-up of the command structure just a few weeks ago to reflect both the troop boost and the huge focus of coalition effort now in both Helmand and Kandahar provinces in the south.
So it does now make military sense to consolidate the British force of around 1,000 troops in Sangin with the main British contingent in central Helmand.
Some may say this demonstrates that the British are at the limits of their commitment. And there is an argument that there needs to be a further increase in troop numbers in Sangin - force density as it is called - which the British are not in a position to supply.
The Prime Minister, David Cameron, has made it clear that there is no question of any further British troop increases in Afghanistan - that is up to others.
It has also been reported that some British commanders have been questioning the strategic value of the continued British commitment in Sangin.
But, from the coalition's perspective, troop numbers will continue to rise. And the US force that is expected to move in to Sangin will probably be bigger than the British force it is expected to replace. How exactly it will operate is another matter.
British and coalition commanders insist that a great deal of progress has been made in Sangin since 2006. But it will remain a combustible mix of very difficult terrain, tribal complexities, links with the opium trade, and the Taliban.
But all that is only part of the potential story. British commanders will be concerned that the move will be perceived as some kind of retreat for the British, that it will be seen as an acknowledgment that the British force is not up to the job in Sangin, in spite of the huge investment made there - especially in terms of lives.
The emotions on this for the military are magnified by the memories of Iraq, and a perception that the British mission there ended in strategic failure, and a desire among commanders to see any repeat of that, or the perception of it. But they argue that the two cases are completely different.
'Impatience and fragility'
But there is the question of what will the public reaction be in the UK? Also, how will allies perceive this shift at what is a critical time for the overall US and Nato-led strategy in Afghanistan?
And will the Taliban be able to portray this as some kind of British, and therefore coalition defeat? After all, perceptions are a huge part of the battle in Afghanistan.
The problem is, as the new US and Nato commander in Afghanistan, General David Petraeus, has acknowledged, the military advantage between the coalition and the Taliban remains in the balance.
It is widely accepted that the next few months will be crucial in terms of whether the coalition can demonstrate progress in its strategy, at a time when coalition government are both wishing to demonstrate continued commitment, but are also conscious of growing impatience and fragility among their own publics. All that amplifies the sensitivities surrounding any change like this.
US and British commanders will argue, and hope, that the changes being unveiled will improve the effectiveness of the overall military effort.