Why Obama did not consider pulling out of Afghanistan

The implosion of General McChrsytal’s career has refocused attention on Afghanistan. Reading Peggy Noonan’s column on the subject I was struck by this paragraph reflecting on Jonathan Alter’s reporting of Obama’s decision making process when he ordered the surge:

More crucially, the president asked policy makers, in Mr. Alter’s words, “If the Taliban took Kabul and controlled Afghanistan, could it link up with Pakistan’s Taliban and threaten command and control of Pakistan’s nuclear weapons?” The answer: Quite possibly yes. Mr. Alter: “Early on, the President eliminated withdrawal (from Afghanistan) as an option, in part because of a new classified study on what would happen to Pakistan’s nuclear arsenal if the Islamabad government fell to the Taliban.”