A third of the UK’s military deaths in Afghanistan since 2001 have been in Sangin UK troops are marking the end of their mission in the Sangin area of Afghanistan’s Helmand province.
Control of the area was formally handed over from UK forces to the US Marine Corps at 0630 BST.
The UK has suffered its heaviest losses in the Sangin area. Of the 337 UK deaths in Afghanistan since 2001, a third have happened there.
Defence Secretary Liam Fox announced in July that the troops were to be replaced by US forces.
After five bloody summers in Sangin, British troops have handed over control to the men of the US Marine Corps and begun pulling out.
A third of all deaths in Afghanistan have taken place here in a district that is as important as it is dangerous.
The Americans have already changed the strategy on the ground, pulling out of some patrol bases that the British fought hard to establish and protect.
The terrain, its location at the cross-section of key routes and a thriving opium trade have made it a key battleground for both the insurgents and British troops.
Military commanders insist this is not a case of America coming to the rescue of beleaguered UK forces but the handover will be a bittersweet moment for the thousands of troops who have fought so hard in Sangin over the last four years.
The UK military has previously insisted the move is a redeployment, now there are more US troops on the ground.
The UK mission in Sangin began in 2006.
The BBC’s Ian Pannell in Kabul said there would be a physical handover, with the union jack lowered and the US flag raised but little would change practically.
He said some members of the 40 Commando Battle Group had already left but the pull-out would be staggered over the coming weeks.
It is a “totemic” moment for the UK, he says, and Sangin is the most dangerous district in Helmand if not the whole of Afghanistan.
On a recent visit to the area, he witnessed a long battle with a number of US soldiers, Afghan soldiers and civilians sustaining injuries.
“Although progress has been made the area remains very difficult, a key battleground for insurgents and coalition forces, and the truth is the Americans will now have to try and finish the job that Britain started,” he added.
Speaking in July, Dr Fox told MPs UK forces had made “good progress” in Sangin, but the move would enable Britain to provide “more manpower and greater focus” on Helmand’s busy central belt, leaving the north and south to the US.
About 300 logistic and security troops – from the Theatre Reserve Battalion stationed in Cyprus – would be sent to Helmand to help with the redeployment, he said.
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