Nato forces have formally taken control of military operations in southern Afghanistan from the US-led coalition which overthrew the Taleban in 2001.
The mission is seen as one of Nato's most challenging
The chief of US-led coalition forces symbolically handed over command at a ceremony in a dusty airfield near the southern city of Kandahar.
British Lt Gen David Richards said the new Nato-led force wanted to deliver "peace, stability and prosperity".
The south has recently been at the centre of an upsurge of violence.
Hundreds of people have been killed in attacks since May in what is the traditional heartland of the Taleban.
This is the first land deployment outside Europe for the Nato forces led by the UK and Canada.
International troop numbers have been building up for months, ahead of the expansion, and there are now double the number of troops in the region.
The 8,000 soldiers will now go under the umbrella of Nato's International Security Assistance Force (Isaf) in six provinces in the south: Day Kundi, Helmand, Kandahar, Nimroz, Uruzgan and Zabul.
"I hope and believe the huge significance of this renewed international commitment will not be lost on the majority who yearn for peace, stability and increased prosperity we came here to deliver," Gen Richards said as he formally took command of the mission.
"These millions of people should be reassured that they will not be let down.
"At the same time, those few thousand who oppose the vast majority of Afghan people and their democratically elected government should note this historic day and should understand they will not be allowed to succeed," he said.
But the BBC's Alastair Leithead in Kandahar says they take control of an area which has seen a dramatic deterioration in security over the past few months.
'Writing's on the wall'
Over the weekend, coalition and Afghan forces said they had killed 20 militants in various clashes in the south and east.
In addition, 10 insurgents were said to have been killed on Sunday in the south-east.
RECENT COALITION CASUALTIES
22 July: Six civilians and two Canadian soldiers killed in double suicide attack in Kandahar
5 July: UK soldier shot dead on patrol near Sangin, Helmand
3 July: Policeman killed in suicide car bombing in Kandahar
1 July: Two UK soldiers killed in rocket-propelled grenade attack in Helmand
27 June: Two British soldiers killed in raid to capture four Taleban members in Sangin, Helmand
15 June: Ten Afghan labourers killed in bomb attack on minibus in Kandahar - the first such major attack on civilians working with US forces
23 May: Three Afghan policemen, 12 Taleban militants killed after ambush in Helmand
In an interview with the BBC, Gen Richards said under his leadership, Isaf forces would be "taking the fight into the depths of the provinces".
He said counter-insurgency efforts were a "critical part of the reconstruction effort", and warned insurgents that "the writing's on the wall... they won't succeed".
Analysts say the failure to rebuild roads, schools and hospitals has fuelled sympathy for the insurgents, and the purpose of this mission is supposed to be to provide security for such reconstruction.
Later this year, Isaf will extend its command across the whole of Afghanistan, by moving into the east of the country.