There is a risk that leaving too few troops after 2014 would stop or stall the already slow development of the Afghan army and police, whose competence - and that of the Afghan government as a whole - is crucial to ending the war successfully.
On the other hand, keeping too many foreign troops beyond 2014 might only prolong Afghanistan's dependence upon them, while Western forces absorb even more casualties. Perhaps the greatest risk is that a wrong calculation by the U.S. on troop levels could enable the Taliban and affiliated insurgents to regain lost territory and influence.
President Barack Obama has pledged to wind down the 11-year-old war, even as Congress presses for an accelerated withdrawal. The intent, approved by NATO in 2010, is to remove combat forces by the end of 2014 but to continue yet-to-be-defined security assistance.
Defense Secretary Leon Panetta has described the broad outlines of a post-2014 plan that amounts to a scaled-down version of what U.S. and NATO forces are already doing: fighting terrorists, training and assisting the Afghan forces, and providing logistical support.
Panetta won't say how many forces would be needed for that set of missions, but analysts estimate as many as 10,000 to 15,000.
Military commanders have laid out options for a post-2014 force ranging from about 6,000 to 15,000, and Panetta and other members of Obama's national security team are debating that issue now, with a decision expected by the end of the year.
But the final number for the end of 2014, and how quickly the military gets to that level, depends on how the White House assesses the political and military risks of having too few troops there to keep the terrorists at bay, or having too many to satisfy war-weary and budget-conscious Americans.
Underlying that debate is perhaps the starkest risk - that by pulling out troops too quickly, Obama would become the president who lost the war and enabled another devastating attack on America.
There are currently about 66,000 U.S. troops in Afghanistan, and commanders would like to maintain as big a force as possible through most of 2013. But others argue that as support for the war continues to erode in Congress and across America, significant cuts must be made at some point next year.
A Pew Research Center poll in early October found that 60 percent of respondents favored removing U.S. troops from Afghanistan as soon as possible, with 35 percent saying they should stay until the country is stable. That's a nearly complete reversal from a September 2008 Pew Research poll that showed 33 percent wanted troops out as soon as possible and 61 percent said they should stay until the country has stabilized.
"You don't want to keep everything in place and then fall off a cliff at the end of 2014," former Pentagon policy chief Michele Flournoy said in an interview with The Associated Press. "You want to gradually step down your residual presence so you have confidence in it, and so you've had a chance to work through some of the issues and challenges that emerge as we go into the latter stages of transition."
Flournoy, who has been mentioned as a possible defense secretary after Panetta steps down, said the military will likely reduce the force in several steps next year, leaving time between cuts to reposition troops.
Any substantial reductions are likely to take place early in the year and again toward the fall, so that the military can maintain a consistent troop strength during the peak fighting season that runs from roughly April to October.
"It's very hard to be repositioning your force as you're fighting. So they'll argue for having a plateau during the fighting season and then taking a steeper drawdown," Flournoy said.
The troop totals also depend on several outside factors, including the commitment of NATO partners and the desires of the Afghan government.
So far, Obama has revealed little of his thinking about the drawdown. But during an October presidential debate he signaled an inclination for a deep reduction, saying, "There's no reason why Americans should die when Afghans are perfectly capable of defending their own country."
Panetta's description this week of the three missions he would like U.S. troops to continue after 2014 suggests a need for a fairly substantial presence.
Some experts argue that the U.S. would have to maintain as many as 30,000 troops in order to continue targeting the terror groups that - if left unchallenged - could regain territory and once again become a threat to the U.S. and other Western nations.
Military analysts Frederick and Kimberly Kagan lay out a case for keeping such a large counterterrorism force, complete with drones, airstrikes and special operations forces bolstered by enough support troops to provide protection on the bases.
Michael O'Hanlon, a fellow at the Brookings Institution in Washington, said that arguments for maintaining a force of 30,000 beyond 2014 are unrealistic in light of other Obama administration priorities. He said it's important to let the Afghans know that the U.S. will not abandon them, but also noted that how much progress Afghan forces make over the next year will also be an important factor.
If they improve, he said, there may be a greater inclination to stick with the mission, but if they don't, "you could ask yourself why do we waste more time, resources and blood over achieving gains that are probably going to be ephemeral and fleeting anyway."
Army Maj. Gen. James Huggins, who returned in September from a one-year tour as commander of allied forces in southern Afghanistan, said fears of abandonment are real.
Huggins recalled a former district governor in the Afghan province of Kandahar, who fought with the U.S.-supported mujahedeen against the Soviets in the 1980s, telling him last year that the Taliban was able to seize power in 1996 because the U.S. abandoned Afghanistan shortly after the Soviet army withdrew in defeat.
At a public symposium on the war on Friday, Huggins said the number of allied troops in his sector fell from a peak of 25,000 to 13,800 as of Oct. 1. He said he was "a little concerned" with that lower number. He did not express a view on the prospect of further reductions in 2013.
Members of Congress want to accelerate the troop withdrawal. The Senate voted 62-33 on a non-binding resolution that would speed up the pullout by an unspecified amount. And while the House voted in May to maintain a force of 68,000, there have been indications more recently that support for that may be faltering.
In September, Rep. C.W. Bill Young, R-Fla., a defense hawk and chairman of the House Appropriations subcommittee that oversees defense spending, said the U.S. should withdraw its forces from Afghanistan.
"We're killing kids who don't need to die," Young said.
(Copyright 2012 by The Associated Press. All Rights Reserved.)