The Foreign Policy Initiative has compiled a fact sheet on Afghanistan, detailing problems with President Obama's withdrawal plan:
What will happen if the United States leaves Afghanistan too quickly?
While significant gains have been made by the United States and its allies in helping the Afghan people to establish a durable state and counter the insurgency, these gains needs to be sustained and furthered. As General David Petraeus, commander of U.S. forces in Afghanistan and the NATO-led International Security Assistance Force, explained to Congress in March 2011: “... [W]e need to ensure that we take a sufficiently long view to ensure that our actions in the months ahead enable long-term achievement in the years ahead."
...A precipitous withdrawal from Afghanistan will imperil the hard-fought and significant gains made by the United States and the NATO-led coalition in recent years. Despite bin Laden’s death and other successes, al-Qaeda, the Taliban and associated forces remain determined both to overthrow the new order that is being established in Afghanistan and to attack the United States and its allies.
Given these threats, defeating this larger network of al-Qaeda, Taliban and associated forces should remain a central objective of U.S. national security policy for the foreseeable future. Success in Afghanistan is crucial to meeting that objective.
Whole thing here.