The decision to withdraw British troops will leave Sangin, the most violent district in Helmand (and probably all of Afghanistan), shows how many parties share in the decision making as well as how hard it is to leave these places without adverse comment.

Essentially the decision was taken by Nato in Afghanistan, with input from the British government. The number two in the International Security Assistance Force (ISAF) set up is a British general, but the main impetus for change came from the Americans.

There are now 18,000 US troops in Helmand (mostly from the US marine Corps) - twice as many as the British have. Increasingly they have taken over sections of the province to the north and south of Britain's Task Force Helmand.

The northern US brigade, Regimental Combat Team 2, has already taken over formerly British districts in Musa Qaleh, Now Zad and Kajaki. It has also taken the British battle group in Sangin under its control.

Announcing today's decision Lieutenant General David Rodriguez, the American who commands ISAF's fighting troops day to day, emphasized that the change was about tidying the boundaries between different commands in southern Afghanistan. As this happens a US Marine battalion will be freed up to the west, and will be able to replace the British in Sangin.

While the creation of orderly boundaries and coherent command structures is of course important to the military mind, it would be foolish to think that this decision had been made without the involvement of the British government. Indeed, announcing the change today defence secretary Liam Fox stressed that it had been, "closely consulted by ISAF".

Subtexts

In deciding what to do with Sangin, there was an emotional subtext as well as a military or political one. For some of the US Marine commanders, the idea that their northern brigade would sub-contract the toughest challenge in Afghanistan to the British sat uneasily with their self image as warriors. They relish the challenge ahead.

For the British military, Sangin aroused even more powerful feelings. The battle group that returned in April, 3 Rifles, lost 30 men killed and had more than 100 wounded - almost half of the entire casualties suffered by Britain's Task Force Helmand over a six month period.

Some British soldiers will be delighted to see the back of the place - with all the myriad dangers it contains. But many of them, particularly the more senior ones, wanted to be able to demonstrate that they had turned the situation around before handing Sangin over to the Americans. There are some indicators of progress, for example in the number of shops open or children at school. However there is an argument locally that the Taliban have colluded in this by letting these places function.

The real problem however is that the security indicators - of attacks on Nato or Afghan forces, and casualties - show no sign of improvement. Indeed, 3 Rifles lost seven more men than the previous battle group.

Security arguments

Some MoD spokesmen have today characterized the hand over of Sangin as being just the same as that of other districts such as Musa Qaleh. But there is a crucial difference. In Musa Qaleh, where Taliban resistance was so strong in 2006 that British troops were actually forced out, the town was re-captured and there has been a dramatic improvement in security. When British troops handed over a few months ago they could therefore say, 'job done'. This is not the case in Sangin, despite their heavy sacrifice.

Some talk about that place as an irredeemable nest of drug traffickers, bomb makers, and jihadists. It may be that the Americans cannot improve the security situation either under the tight timescales demanded by their political masters.

All we can say for the moment is that leaving Sangin ought to lead to a significant reduction in British casualties and that may make it possible for the UK to soldier on in Afghanistan for longer than might otherwise have been the case. Those who hoped to hand the place on to the US or Afghans with much greater security are unlikely to achieve this by October, when the changeover will occur. So the arguments are bound to swirl about whether the British have done a 'Basra 2' and left the Americans to finish the job. People will also ask whether both the US military and the government in London ultimately ran out of the patience required to let them do that.