In 2015, the Obama administration encouraged Afghan commanders to give up defending some of the most remote checkpoints and outposts that were seen as difficult to reclaim and hold. General Nicholson supported the idea after he took command in 2016, the official said.

Should Afghan Troops pull back now, defending remote pockets of the country would mostly be left to the local police, which are more poorly trained than the military and far more vulnerable to Taliban violence. In some areas, police officers have cut deals with the Taliban to protect themselves from attacks.

Ghulam Sarwar Haidari, the former deputy police chief of northwestern Badghis Province, said his forces withdrew from the small town of Dara-e-bom after the Afghan National Army abandoned their outposts in past months. “We should lose 100 lives to retake that area,” he said.

Not all of the roughly 14,000 United States troops currently in Afghanistan have pulled back to cities. Some who are training and advising Afghan troops as part of Mr. Trump’s war strategy are stationed in bases in remote areas and smaller towns.

Mr. Trump has long called for ending the war in Afghanistan and only reluctantly accepted Defense Secretary Jim Mattis’s advice to send an additional 4,000 troops in an attempt to claim victory.

The Trump administration is also instructing top American diplomats to seek direct talks with the Taliban to refuel negotiations to end the war, and two senior Taliban officials said on Saturday that such talks had been held in Qatar a week ago. If they happen, the negotiations would be a major shift in American policy and would serve as a bridge to an eventual withdrawal of United States forces from Afghanistan.

Evan McAllister, a former reconnaissance Marine staff sergeant and sniper, fought in parts of Helmand Province in 2008 and 2011 — areas that are now almost entirely under Taliban control. He said trying to maintain an Afghan government-friendly presence in rural areas was, and still is, a “fool’s errand.”

“Attempting to control rural areas in Afghanistan always eventually ends up boiling down to simple personal survival,” Mr. McAllister said. “No strategic gains are accomplished, no populace is influenced, but the death or dismemberment of American and Afghan troops is permanent and guaranteed.”