Ahmad Behzad, an influential member of Parliament from Herat Province, said that a full withdrawal would mean the reversal of every significant achievement of the past 12 years of American presence. But he said he did not think that would happen. “The United States and NATO strategists know that a full withdrawal in the long term is not going to be in the interests of either side,” he said.

Beyond that, Mr. Behzad said, Mr. Karzai will face a huge backlash from the Afghan public if he does not sign the security deal. “People will pour into the streets and force him to sign,” he said.

Each side has still left itself a way out. Ms. Rice did not say the Americans would definitely walk away after 2014, only that they would begin planning for the possibility.

Mr. Karzai did say he would sign, but suggested not until after the presidential elections, next April. Many close to him still think he might still sign earlier, taking political cover by bowing to the loya jirga’s decision.

Others wondered if it would really be as impossible as the Americans claim to plan for a long-term presence of 8,000 to 12,000 if the security agreement were delayed until April.

Perhaps more than any logistical complication, however, political currents within the United States might add to pressure for a complete pullout, despite American military commanders’ desire for lasting bases in Afghanistan. Recent polls in the United States show more resistance than not to the idea of keeping American forces there.

“He doesn’t realize how little support there is in America toward Afghanistan; he doesn’t realize how critical this is for the Afghan government,” Mr. Mujadidi said. “The Americans are the winners in this game, because what America has to lose is far less than what Afghanistan has to lose.”