On the eve of Saturday's ceremony to sign an agreement with the United States to begin peace negotiations, the Taliban declared it had already won. "This is a day of victory," said Sher Mohammad Abbas Stanekzai, a Taliban negotiator. "Victory has come with the help of God."

In one sense, this crowing seems premature. While it's true that the U.S. intends to reduce its troop presence from more than 12,000 to 8,600, further withdrawals are conditioned on the Taliban adhering to its commitments to reduce violence and sever ties with al-Qaida. As Defense Secretary Mark Esper said in Kabul on Saturday, the U.S. "would not hesitate to nullify the agreement" if the Taliban reneges on its commitments.

Another condition for the Taliban will be to enter negotiations for a final peace settlement among Afghan groups themselves. That has always been a red line for the Taliban, which has never recognized the legitimacy of the elected government in Kabul. So why has it agreed to these talks now?