NATO took over eastern Afghanistan from U.S.-led forces Thursday, assuming control of 12,000 American troops and extending its military role to the entire country.

The commander of the NATO-led force, British Gen. David Richards, who was promoted to a four-star general Thursday, called the move "historic" in a ceremony also attended by Afghan President Hamid Karzai and U.S. Lt. Gen. Karl Eikenberry.

The handover "illustrates the enduring commitment of NATO and its international partners to the future of this great country," said Richards, who now holds the British military's highest rank.

Richards all but guaranteed progress in Afghanistan's deteriorating security, joking that he would appear in front of a firing squad if the country isn't safer by the end of his command in February.

"If by next spring these improvements are not evident then I will be surrendering to whoever wants to put me up against a wall," he told reporters after the ceremony.

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With about 12,000 troops, the U.S. is the biggest contributor to the 31,000-strong NATO mission. Britain has 5,200 troops and Germany has 2,750 troops in the 37-nation force.

"A key point to remember in this transition is that the United States maintains its full commitment to Afghanistan," Eikenberry said. "As a NATO member, the United States will remain by far the single-largest contributor of troops and military capability."

Eikenberry will continue to command some 8,000 U.S. troops functioning outside NATO who are tracking Al Qaeda terrorists, helping train Afghan security forces and doing reconstruction work.

Eikenberry also retains administrative and legal responsibility for U.S. forces under NATO, overseeing matters such as logistics and military justice, said U.S. military spokesman Col. Tom Collins.

Eikenberry said that consolidating the command under Richards streamlines western troops' effectiveness. It confines direct U.S. control to a single chief enclave: the sprawling American base at Bagram. Most air operations in the Afghan theater also remain under American oversight.

U.S.-operated prisons and interrogation centers at Bagram will remain under U.S. 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.

A 10-day NATO offensive in Kandahar province last month saw hundreds of Taliban fighters killed and more than 100 taken prisoner. Few NATO soldiers were killed or wounded. Richards said the operation prevented the Taliban from holding valuable ground that might have allowed it to attack Kandahar, Afghanistan's second-largest city.

"The significant defeat of the Taliban is probably the greatest single defeat they suffered since 2001," Richards said. "We have unequivocally proved our mettle in military operations."

The NATO takeover, which came months ahead of schedule, 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.

An American four-star general, Army Gen. Dan K. McNeil, will take charge of both U.S. and NATO forces in February, pending confirmation by the Senate.

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