Created by President John F. Kennedy in the 1960s, the Green Berets have as one of their core missions what is called “foreign internal defense” — using combat, mentoring, language and cross-cultural skills to train local forces in rugged environments, as they are today in missions conducted quietly in dozens of nations around the world.

Just as significant would be what the American military’s conventional forces stop doing.

Americans would no longer be carrying out large numbers of patrols to clear vast areas of Afghanistan of insurgents, or holding villages and towns vulnerable to militant attacks while local forces and government agencies rebuilt the local economy and empowered local governments.

Those tasks would fall to Afghan forces, with Special Forces soldiers remaining in the field to guide them. This shift has already begun to take place.

The defense secretary, Leon E. Panetta, surprised NATO allies last week when he announced that American forces would step back from a leading role in combat missions by mid-2013, turning over security responsibilities to Afghan forces a year earlier than expected. The description of the shift to a Special Operations mission in Afghanistan by senior officials, who spoke on the condition of anonymity because the final plans have not been approved, go a long way toward explaining what Mr. Panetta sketched out for the allies.

White House officials confirmed in broad terms the shift to a Special Operations mission, and said a formal announcement on the future of the mission was expected at the May summit meeting of NATO leaders in Chicago.

“The president said in June that when the drawdown of surge forces is complete in September, U.S. troops will continue coming home at a steady pace and our mission will shift from combat to support as the Afghans take the lead,” said Tommy Vietor, the National Security Council spokesman.

The United States has about 90,000 troops in Afghanistan, with 22,000 of them expected to leave by this fall. No schedule has been set for the pace of withdrawal for the 68,000 American troops who will remain, although some administration officials are advocating for Mr. Obama to order another reduction by the summer of 2013.