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 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.

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.

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.