Chapter 8, Verses 6 and 7 - Sun Tzu

So, the student of war who is unversed in the art of war of varying his plans, even though he be acquainted with the Five Advantages, will fail to make the best use of his men. Hence in the wise leader's plans, considerations of advantage and of disadvantage will be blended together.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.