NATO on Thursday was set to expand its security mission to the whole of Afghanistan, taking command of more than 13,000 US troops in the east of the country.
British Lt Gen David Richards, commander of NATO's International Security Assistance Force, takes charge of US-coalition troops currently led by US Lt Gen Karl Eikenberry.
Eikenberry called the handover, due to take place at a ceremony in Kabul Thursday, a "historical day" that shows the commitment of NATO to the Afghan people.
"The people of Afghanistan should know that NATO and ISAF, under the command of Gen Richards, has the ability to maintain peace all over the country," Eikenberry said in prepared remarks distributed before the ceremony.
Of the 41,000 foreign troops in Afghanistan, only 8,000 US troops will function outside NATO control: Those tracking Al-Qaeda terrorists or involved in air operations.
The overall level of American forces will remain around 21,000.
The command consolidation confines direct US control to a single chief enclave: The sprawling American base at Bagram.
A US Army helicopter unit based at Kandahar airfield also will remain under American oversight.
US-operated prisons and interrogation centers at Bagram will remain under US command, while NATO will continue to transfer its detainees to Afghan police.
The alliance's troops took command of southern Afghanistan just two months ago and have struggled to stem escalating violence there.
It also has troops in the north and west of the country and patrols the capital, Kabul.
The NATO takeover caps an already historic expansion of missions for the largely European alliance that was created as a Cold War bulwark against the Soviet Union.
Its combat role in southern Afghanistan is the largest the alliance has ever undertaken.
The Taliban have recently staged an unexpected resurgence and stepped up attacks, triggering major battles that have left more than a thousand dead in the past few months.
The move leaves Eikenberry's role in doubt. ISAF spokesman Maj Luke Knittig said he may remain in Afghanistan but under the auspices of the US Embassy.