Those within the administration who have suggested limiting large-scale U.S. ground combat in Afghanistan, including Vice President Biden, have pointed to an improved counterterrorism effort as evidence that Obama’s principal objective — destroying al-Qaeda — can be achieved without an expanded troop presence.

The most important new weapon in the Western arsenal is said to be the recruitment of spies inside al-Qaeda and affiliated organizations, a long-sought objective. “Human sources have begun to produce results,” Richard Barrett, head of the United Nations’ al-Qaeda and Taliban monitoring group, said Tuesday. Barrett is the former chief of Britain’s overseas counterterrorism operations.

Current and former senior U.S. officials, who spoke about intelligence matters on the condition of anonymity, confirmed what one former CIA official called “our penetration of al-Qaeda.” A senior administration official said that success had come “because of, first of all, very good intelligence capabilities . . . to locate and identify individuals who are part of the al-Qaeda organization.”