MA in Democracy and Governance co-director Dan Brumberg is pessimistic about Afghanistan.  Brumberg writes,

…can the White House achieve its goals with 30,000 to 35,000 more troops (including NATO)? Here there is reason for concern. The White House’s implicit optimism rests partly on the assumption that a U.S.-led military surge in Afghanistan will produce some of the security and political benefits that came with the previous surge in Iraq…the substantial differences between the two countries suggest that the gains achieved in Iraq will be hard to replicate in Afghanistan.

The main reason Brumberg thinks the analogy does not hold is that in Iraq, the majority Shi’ite supported the US and resistance came from a minority group, the Sunni.  By contrast, in Afghanistan, the largest ethnic group, the Pashtun, do not support the US.  Rather, ethnic minorities, such as Hazara and Uzbek, support the US.  Hence, in Iraq, victory for the US meant the majority defeating the minority, while in Afghanistan, victory for the US requires an alliance of minority groups defeating the largest group.  The latter is far more difficult than the former, according to Brumberg.  Sounds reasonable to me.