Instead of embracing General McChrystal's proposal for a substantial increase in the U.S. military presence--or even adopting a "McChrystal-Light" strategy--the Obama administration should begin a phased withdrawal of troops over the next 18 months, retaining only a small military footprint relying on special forces personnel. Otherwise, America will be entangled for years--or decades--in pursuit of unattainable goals.

We need to "define success down" in Afghanistan. That means abandoning any notion of transforming ethnically fractured, pre-industrial Afghanistan into a modern, cohesive nation state. It also means reversing the drift in Washington's strategy over the past eight years that has gradually made the Taliban (a parochial Pashtun insurgent movement), rather than al Qaeda, America's primary enemy in Afghanistan. A more modest and realistic strategy means even abandoning the goal of a definitive victory over al Qaeda itself.

Instead, we need to treat the terrorist threat that al Qaeda poses as a chronic, but manageable, security problem. Foreign policy, like domestic politics, is the art of the possible. Containing and weakening al Qaeda may be possible, but sustaining a large-scale, long-term occupation of Afghanistan and creating a modern, democratic country is not.