In a major milestone toward ending a decade of war in Afghanistan, Defense Secretary Leon E. Panetta said on Wednesday that American forces would step back from a combat role there as early as mid-2013, more than a year before all American troops are scheduled to come home…

Promising the end of the American combat mission in Afghanistan next year would also give Mr. Obama a certain applause line in his re-election stump speech this year.

Mr. Panetta said no decisions had been made about the number of American troops to be withdrawn in 2013, and he made clear that substantial fighting lies ahead. “It doesn’t mean that we’re not going to be combat-ready; we will be, because we always have to be in order to defend ourselves,” he told reporters on his plane on his way to a NATO meeting in Brussels, where Afghanistan is to be a central focus…

Mr. Panetta offered no details of what stepping back from combat would mean, saying only that the troops would move into an “advise and assist” role to Afghanistan’s security forces. Such definitions are typically murky, particularly in a country like Afghanistan, where American forces are spread widely among small bases across the desert, farmland and mountains, and where the native security forces have a mixed record of success at best.