BRUSSELS — Nearly three months after President Trump announced his new strategy for the war in Afghanistan, the United States and its international allies are still trying to come up with the troops required to carry it out.

The relatively few American and international troops in Afghanistan are charged with training the Afghan military and with helping them beat back Taliban forces. With the militants at their strongest level since the start of the war in 2001, the issue of troop levels has renewed importance.

“We’ve fought most of the year,” Gen. John W. Nicholson Jr., the commanding officer of American and international forces in Afghanistan, told reporters in Brussels on Wednesday, “however, at the lowest level of capability that we’ve ever had in the past 16 years.”

Since February, General Nicholson has requested several thousand additional troops for the mission, to break what he calls a stalemate in the Afghan conflict. Mr. Trump announced the new strategy for Afghanistan in August, and authorized an American troop increase of roughly 3,000.