Long wars are antithetical to democracy. Protracted conflict introduces toxins that inexorably corrode the values of popular government.
I hope that we get at least one great benefit from the war in Afghanistan, since it looks like we'll get little else: the lesson that even "good" wars are bad. Aside from the trite costs "in blood and treasure," war creates new enemies even when it vanquishes the old ones, damages our reputation abroad, desensitizes our people to violence (which carries its own attendant degradations of character), and breeds resentment of our democratic values and civil rights among both the military and civilian populations. It makes us coarser and more authoritarian as a people.