“If we withdraw, and the international community withdraws its aid, you will see the potential for the Afghan government to collapse, the insurgency to gain momentum and territory, take over eastern Afghanistan, recreating a safe haven for terrorist elements that still harbor an anti-U.S. agenda,” said Michèle Flournoy, a former top Pentagon official in the Obama administration. “After all of this effort and all of this sacrifice and all of this progress, you’re back to a new safe haven for terrorists? It’s like, it just makes no sense.”

American intelligence officials have warned in classified assessments that insurgents could retake key areas of Afghanistan in the south and the east in as little as a year after American troops are fully withdrawn. The assessments also warn that Kabul, the capital, could quickly come under more serious attack than it has in recent years.

Such a turn could also lead to insecurity for India and Pakistan, foreign policy experts say, with each of the nuclear-armed South Asian nations entering a more aggressive proxy war in Afghanistan in a bid for regional influence and a trump card to play against the other.

“The neighboring countries, they all want us to stay,” said David Sedney, former deputy assistant secretary of defense for Afghanistan, Pakistan and Central Asia. “If we were to leave, the consequences for Pakistan, for the Indians — these countries want a stable Afghanistan.”

American planners are thinking about what would happen if the United States is forced into a full and final troop withdrawal. Part of that contingency planning, officials say, will include looking to other countries — perhaps in Central Asia — for air bases that would allow continued drone operations in the region.

Defense Secretary Chuck Hagel alluded to that this month when he said at a news conference that the military was “constantly updating” where to put drone bases, “where the threats are most significant, where do you have allies that are willing to work with you.”

But military officials say that a complete pullout from Afghanistan, where the United States now has the luxury of the base at Bagram from which to provide support, would hurt the American counterterrorism effort.