Chapter 10, Verses 21 and 22 - Sun Tzu

The natural formation of the country is the soldier's best ally; but a power of estimating the adversary, of controlling the forces of victory, and of shrewdly calculating difficulties, dangers and distances, constitutes the test of a great general. He who knows these things, and in fighting puts his knowledge into practice, will win his battles. He who knows them not, nor practices them, will surely be defeated.
~ from The Art of War, Giles translation ~

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