Updated 3:44 p.m. ET
A senior administration official said this afternoon that President Barack Obama will tell the American people Tuesday night that U.S. troops will start leaving Afghanistan in July 2011, before the end of his first term.
The official stressed, however, that the date marks the beginning of a process that does not have a defined endpoint. The full withdrawal of troops, the official said, will depend on conditions on the ground.
The rough outlines of a withdrawal plan will be part of the revamped Afghanistan strategy that Mr. Obama is announcing in a speech from West Point at 8:00 p.m. Eastern Time. He will tell Americans he is sending 30,000 more troops to Afghanistan over the next six months, an accelerated war escalation that would have the first Marines there as early as Christmas, senior administration officials said.
With the full complement of troops to be deployed by next summer, the heightened pace of Mr. Obama's military deployment in the 8-year-old war would appear to match the 2007 troop surge in Iraq, which rushed 20,000 combat forces to quell violence there. The Afghan surge would similarly aim to reverse Taliban gains and secure population centers in the volatile south and east parts of the country.
In his prime-time speech to the nation, Mr. Obama will tie the escalation to an exit strategy, laying out a rough timeframe for when the main U.S. military mission would end, and outline plans to accelerate handing over security responsibilities to Afghan forces.
Mr. Obama will try to sell a skeptical public on his bigger, costlier war plan by coupling the large new troop infusion with an emphasis on stepped-up training for Afghan forces that he says will allow the U.S. to leave.
Public opinion in this country has become increasingly divided over American participation in the stalemated war.
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The new infusion of troops had been envisioned to take place over a year, or even more, because force deployments in Iraq and elsewhere make it logistically difficult, if not impossible, to go faster.
Instead, Mr. Obama directed his military planners to make the changes necessary to speed up the Afghanistan additions, officials said, speaking on condition of anonymity because the details had not yet been announced. They said the option the president has chosen gets more troops into Afghanistan faster than any other option that had been presented to him.
Military officials said at least one group of Marines is expected to deploy within two or three weeks of Obama's announcement, and would be in Afghanistan by Christmas. Larger deployments wouldn't be able to follow until early in 2010.
The initial infusion is a recognition by the administration that something tangible needs to happen quickly, officials said. The quick addition of Marines would provide badly needed reinforcements to those fighting against Taliban gains in the southern Helmand province, and could lend reassurance to both Afghans and the U.S. public.
Speaking to "Early Show" co-anchor Harry Smith Tuesday morning, White House press secretary Robert Gibbs said the months-long process of deliberation Mr. Obama took to make his final decision was necessary.
"Everybody involved really worked hard with the president to make this policy better than it would have been had we announced it after only a week," said Gibbs. (More from the Gibbs interview)
Thirty thousand more troops would be 10,000 fewer than Gen. Stanley McChrystal, the commander in Afghanistan, requested, reports CBS News correspondent David Martin. The president hopes to make up at least some of the difference with .
"This is going to be an international effort," Gibbs told "the Early Show". "This is not one country, or one region of the world's problem."
"I think NATO will come through with a couple thousand and so I think we'll still be somewhat short of what Gen. McChrystal proposed," Michael O'Hanlon of the Brookings Institution told CBS News.
On a conference call with reporters this afternoon, senior administration officials said that NATO would "step forward with additional contributions of troops," though they did not say how many. There are currently 40,000 troops from countries other than the United States in Afghanistan, along with 68,000 U.S. troops.
Martin reports that the commandant of the Marine Corps has said his troops will among be the first in - about 9,000 into the Taliban heartland in Southern Afghanistan. It was not immediately clear whether that contingent of 9,000 would be deployed by the end of 2009, or would be staggered.
Mr. Obama will discuss a specific withdrawal scenario in his speech tonight, according to administration officials.
"We want to - as quickly as possible - transition the security of the Afghan people over to those national security forces in Afghanistan," Gibbs told ABC News. "This can't be nation-building. It can't be an open-ended, forever commitment."
In Kabul, Lt. Gen. William B. Caldwell, the new head of a U.S.-NATO command responsible for training and developing Afghan soldiers and police, said Tuesday that although the groundwork is being laid to expand the Afghan National Army beyond the current target of 134,000 troops, to be reached by Oct. 31, 2010, no fixed higher target is set.
There is a notional goal of eventually fielding 240,000 soldiers and 160,000 police, but Caldwell said that could change.
"Although that is a goal and where we think it could eventually go to, it's not a hard, firm, fixed number," he said in a telephone interview with The Associated Press.
Mr. Obama began rolling out his decision Sunday night, informing key administration officials, military advisers and foreign allies in a series of private meetings and phone calls that stretched into Monday. In the past 24 hours he has spoken to Afghan President Hamid Karzai, Pakistan President Asif Ali Zardari and others about his plans.
Mr. Obama's announcement comes near the end of a year in which the war has worsened despite Mr. Obama's previous infusion of 21,000 forces.
With U.S. casualties in Afghanistan sharply increasing and little sign of progress, the war Mr. Obama once liked to call one "of necessity," not choice, has grown less popular with the public and within his own Democratic party. In recent days, leading Democrats have talked of setting tough conditions on deeper U.S. involvement, or even staging outright opposition.
Mr. Obama also deliver a deeper explanation of why the U.S. must continue to fight more than eight years after the war's start in his address, emphasizing that Afghan security forces need more time, more schooling and more U.S. combat backup to be up to the job on their own. He will make tougher demands on the governments of Pakistan and, especially, Afghanistan, and will provide a fresh path toward disengagement.
In Afghanistan, rampant government corruption and inefficiency have made U.S. success much harder. Mr. Obama was expected to place tough conditions on Karzai's government, along with endorsing a stepped-up training program for the Afghan armed forces along the outline recommended this fall by U.S. trainers.
The displeasure on both sides of the aisle was likely to be on display when congressional hearings on Mr. Obama's strategy get under way later in the week on Capitol Hill.
A briefing for dozens of key lawmakers was planned for Tuesday afternoon, just before Mr. Obama was set to leave the White House for the speech against a military backdrop at West Point.
Military officials said the speech is expected to include several references to Iraq, where the United States still has more than 100,000 forces. The strain of maintaining that overseas war machine has stretched the Army and Marine Corps and limited Mr. Obama's options.
You can watch the speech on your CBS station at 8 p.m. ET or online at CBSNews.com.