The US must convince its Afghan allies of its commitment to developing a stable nation. That can't happen without US public support.
Recent polls show that a majority of Americans believe the war in Afghanistan is not worth fighting. This is said to weigh heavily on President Obama as he considers Gen. Stanley McChrystal's request to focus on counterinsurgency and add 40,000 troops to the field.
Ideally, when leaders deliberate over a proposed foreign policy, they consider whether it furthers the national interest, not whether the public supports it at the moment. A paradox of US democracy is that people expect their officials to ignore them from time to time.
Leaders generally depend on two assumptions when making foreign-policy decisions: (1) that the public will support what emerges as good policy over the long term, and (2) that a foreign policy's effectiveness is divorced from domestic public support.
Under normal circumstances, the public could expect Mr. Obama to focus only on the question of whether the war's goals are worth its costs, ignoring the transitory polls.
The situation in Afghanistan, however, throws this model out the window. Today, the American public is the newest front in winning the conflict in Afghanistan.
Obama's consideration of public opinion shows that the administration recognizes that the public might not support even a successful long-term effort, and this lack of support might doom an otherwise effective mission.
What are some of the reasons for lagging support?
Because the war's goals are vague and abstract (e.g., building democracy and stability) and difficult to measure (e.g., making terrorism less likely), the public might deem any long-term mission obscure and wasteful – even an effective one.