Mrs. Clinton said the “best information available at the moment” pointed to July 2011 as the pivotal moment. But she said that the withdrawal of troops would go on “probably for the foreseeable future,” as would requests for “continuing logistical support for the Afghan security force.”

Moreover, Mrs. Clinton said, despite the “limited” duration of the American military presence in Afghanistan, “our civilian commitment must continue, even as our troops begin coming home.”

“We will be asking the young men and women, who not only serve in the military but are part of our civilian service team, to be taking great risks and facing extraordinary sacrifices,” she said, adding that “we will do everything we can to ensure that their sacrifices make our nation safer.”

As they have before, Mrs. Clinton and the Pentagon officials asserted, sometimes in the face of skeptics, that problems in Afghanistan and Pakistan must be dealt with together, and that terrorists from Al Qaeda and the Taliban insurgents are part of the same threat, even if they do not always coordinate their efforts.

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The officials asserted, too, that the plans to withdraw American troops were definite enough, with the July 2011 starting date, to pressure the Afghan government to assume its responsibilities, yet flexible enough to meet the needs of the American military.

“It’s not arbitrary at all,” Admiral Mullen said, calling 2011 the right time to begin the transition to full Afghan control “responsibly and based on conditions.”

Some members of Congress, notably Senator John McCain , the ranking Republican on the Senate Armed Services Committee, have argued that it is unwise to set even an initial withdrawal date because doing so encourages an enemy. But Senator John Kerry , Democrat of Massachusetts and chairman of the foreign relations panel, disagreed. Setting a target date “will help create a sense of urgency” among Afghans, Mr. Kerry said.

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“And for the Afghans who chafe at foreign boots on their soil, it sends a message that while America will remain committed to the Afghan people, we aren’t interested in a permanent occupation,” Mr. Kerry said.

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Senator Jim Webb , Democrat of Virginia , wanted to know just what “conditions on the ground” would indicate that the withdrawal process could begin.

Mrs. Clinton said, “I think you raised a very profoundly important question.” Part of the answer, she said, is “decentralization of government’s function and authority” to reflect the way the mountainous, mostly rural country actually operates.

The Obama administration has tried to distinguish its efforts in Afghanistan from what it has characterized as the Bush administration’s overly ambitious goals to build a democratic government in Iraq . Mr. Kerry said he was pleased that the approach to Afghanistan that Mr. Obama laid out did not amount to “an open-ended nation-building exercise or a nationwide counterinsurgency campaign.”

But the difference between engaging in “nation-building” and propping up a fledgling Afghan government may not always be easy to discern. For instance, Mrs. Clinton said that America’s commitment to Afghanistan was reflected not only by the presence of American troops but also by the significant commitment of American civilians in the country.

“Civilian experts and advisers are helping to craft policy inside government ministries,” Mrs. Clinton said, adding that financial aid for those ministries would not be released until American overseers had confidence in them.

At another point, Admiral Mullen noted that the development of a reliable Afghan Army had been slow because “the Taliban make a lot more money than the national security forces right now.” (The opium trade is a primary source of income for the Taliban.)