WASHINGTON (CNN) - Determining the amount of troops necessary to win a war is never an easy decision for a commander in chief and his military commanders if history is any guide.
And it's a dilemma President Obama facesas pressure mounts on him to decide what strategy will improve conditions in Afghanistan.
The president and his top military, national security and foreign policy advisers are conducting an intensive strategic review of the U.S. military presence in the war-torn nation.
Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, is calling for a counterinsurgency strategy that would add as many as 40,000 troops.
But others in the administration want a different approach.
Vice President Joe Biden has called for a counterterrorism strategy, which would focus on using special forces and technology to reduce the number of al Qaeda insurgents on the Afghanistan-Pakistan border.
If the president should listen to McChrystal and adopt a troop "surge," the question remains: How many is enough?
One expert said such a large number is needed to reduce violence throughout the country - the 40,000 troops would allow the U.S. military to "reverse the momentum of the insurgency, which has been on the rise," said Kimberly Kagan of the Institute for the Study of War, who has advised McChrystal on Afghanistan.
That number, Kagan said, would help fill in gaps around Kandahar in the southern part of the country where Taliban forces have amassed. But she warned that eventually, troops would also be needed to tamp down the insurgency in other parts of the country.