First, from SunTzu, the ancient Chinese military philosopher and author of "The Art of War":
If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat.
Second, from General Stanley McChrystal (the U.S. commander in Afghanistan), during Congressional testimony on Tuesday, December 8th (also as quoted by Slate's Fred Kaplan):
There is much in Afghanistan that I do not understand.