CAMP LEATHERNECK, Afghanistan — U.S. Marines will march out of Afghanistan by the thousands next year, winding down combat in the Taliban heartland and testing the U.S. view that Afghan forces are capable of leading the fight against a battered but not yet beaten insurgency in the country’s southwestern reaches, senior U.S. military officers say.

At the same time, U.S. reinforcements will be sent to eastern Afghanistan in a bid to reverse recent gains by insurgents targeting Kabul, the capital.

Gen. James F. Amos, commandant of the Marine Corps, said in an Associated Press interview that the number of Marines in Helmand province will drop “markedly” in 2012, and the role of those who stay will shift from countering the insurgency to training and advising the Afghan security forces.

The change suggests an early exit from Afghanistan for the Marine Corps, even as the prospects for solidifying their recent successes are uncertain.

“Am I OK with that? The answer is ‘yes,'” Amos said. “We can’t stay in Afghanistan forever.”

He added: “Will it work? I don’t know.”

At stake is President Barack Obama’s pledge to win in Afghanistan — the war he touted during his 2008 presidential campaign as worth fighting, while pledging to get out of Iraq. Facing a stalemate in 2009, Obama ordered an extra 30,000 U.S. troops to Afghanistan — including about 10,000 Marines to Helmand province — on the belief that if the Taliban were to retake the government al-Qaida would soon return to the land from which it plotted the 9/11 attacks.

Also at stake are the sacrifices of the nearly 300 Marines killed in Afghanistan over the past three years.

Weighing against prolonging the conflict is its unsustainable cost and what author and former Defense Department official Bing West has called its “grinding inconclusiveness.”

In a series of pep talks to Marines in Helmand this past week, Amos said the Marine mission in Afghanistan would end in the next 12 to 18 months. That is as much as two years before the December 2014 deadline, announced a year ago, for all U.S. and other foreign troops to leave the country.

“Savor being out here together,” Amos told Marines on Thanksgiving at an outpost along the Helmand River called Fiddler’s Green, “because it’s going to be over” soon.

He was referring only to the Marines’ role, which is limited mainly to Helmand, although there also are Marine special operations forces in western Afghanistan. The U.S. military efforts in Kandahar province and throughout the volatile eastern region are led by the Army, along with allied forces.

Helmand and neighboring Kandahar for the past two years have been the main focus of the U.S.-led effort to turn the tide against a resilient Taliban insurgency. In that period, the Taliban and other insurgent networks have grown bolder and more violent in Afghanistan’s eastern provinces where they have the advantage of sanctuary across the border in Pakistan and where U.S. and NATO forces are spread more thinly than in the south.

During two days of visiting Marine outposts throughout Helmand this week, Amos touted progress against the Taliban and was told by Marine commanders that plans are well under way to close U.S. bases, ship war equipment home and prepare for a major drawdown of Marines beginning next summer. Amos declined to discuss the number of Marines expected to leave in 2012, but indications are that 10,000 or more may depart.

There are now about 19,400 Marines in Helmand, and that is due to fall to about 18,500 by the end of this year.

Marine Gen. John Allen, the top overall commander of U.S. and NATO forces in Afghanistan, was ordered by Obama last summer to pull out 10,000 U.S. forces by the end of this year and another 23,000 by the end of September 2012. That has driven the move to accelerate a transition to Afghan control.

Allen said in an interview Thursday that winding down the Marine combat mission in Helmand makes sense because security “has gotten so much better now.” He said the pullout of 23,000 U.S. forces in 2012, including an unspecified number of Marines, likely will begin in the summer, which historically is the height of the fighting season in Afghanistan. Allen said Afghan security forces, often criticized for weak battlefield performance, desertion and a lack of will, are closer to being ready to assume lead responsibility for their nation’s defense than many people believe.

“The Afghan national security forces are better than they thought they were, and they’re better than we thought they were,” Allen said.

That is why he thinks it’s safe to lessen the Marine’s combat role in Helmand, reduce their numbers and put the Afghans in charge.

That approach also allows Allen to build up elsewhere. He said that in 2012 he will put more U.S. forces in eastern Afghanistan, increase the number of U.S. special operations forces who are playing a key role in developing Afghan forces, and add intelligence, surveillance and reconnaissance resources. He said he plans to add “several battalions” of U.S. forces in the east. He gave no specific troop number, but a battalion usually totals about 750.

“I’m going to put a lot more forces and capabilities into the east,” he said. “The east is going to need some additional forces because our intent is to expand the security zone around Kabul.”

The top Marine in Helmand, Maj. Gen. John Toolan, said he is not convinced that 2012 is the best time to shift the focus to eastern Afghanistan, where the Haqqani network has taken credit for a series of spectacular attacks recently, including suicide bombings inside Kabul, the heavily secured capital. He said he believes the Taliban movement in southern Afghanistan is still the biggest threat to the viability of the central government.

Toolan said the Marines continue to make important progress against a Taliban whose leaders are showing signs of frustration and division.

“They’re starting to break up,” Toolan said. “There’s still a lot to be done to see that these insurgents stay on their backs.”

Stephen Biddle, a defense analyst at the Council on Foreign Relations and who recently visited U.S. forces in Afghanistan, said there is a risk to putting the Afghans in the lead role in Helmand as early as 2012.

“If you throw them into the deep end and put them in the lead in really tough neighborhoods you run the risk that they get their noses bloodied early in ways that could make it hard for them to recover because they lose confidence,” Biddle said in an interview in Washington. On the other hand, if the U.S. and its allies wait until 2013 or 2014 to hand off to the Afghans in the most challenging areas, there would be less chance to bail them out.

“It’s a dilemma with no obvious solution to it,” he said.