If Mr. Trump does not grant the Pentagon’s request for a more robust American troop level, there is concern that the Afghan forces may not be able to even maintain the current stalemate, causing the country’s security situation to backslide further, and that Kabul, the capital, could come under more high-visibility attacks.

Even if the extra numbers do come through, military officials acknowledged that Afghanistan’s security forces were nowhere close to being able to defend the country on their own or to keep it from returning to its former status as a launching pad for terrorist attacks abroad.

A resurgent Taliban, which by American estimates controls more than 40 percent of the population centers of Helmand Province in the south, continues to bedevil the beleaguered Afghan security forces, while the Islamic State has taken root in Nangarhar Province in the east.

Despite the challenges, Gen. Joseph L. Votel, the head of United States Central Command, and other senior officers presented an upbeat assessment of the war effort in a series of briefings with reporters during a visit to Afghanistan to meet with the country’s military and political leaders.

The Taliban had not managed to take any provincial capitals or to hold any important cities in this year’s fighting season, they said.

“The enemy is not doing very well,” said Maj. Gen. James B. Linder, the American Special Operations commander in Afghanistan. “The reality on the ground,” he added, “is quite different” from what the Taliban’s claims may suggest.

In an attack last month at the Iraqi Embassy in Kabul, only two civilians were killed, military officials said, noting that the attack could have been much worse were it not for the fast response of the Afghan police special operations forces.