Chapter 9, Verses 42 and 43 - Sun Tzu

If soldiers are punished before they have grown attached to you, they will not prove submissive; and, unless submissive, then will be practically useless. If, when the soldiers have become attached to you, punishments are not enforced, they will still be unless. Therefore soldiers must be treated in the first instance with humanity, but kept under control by means of iron discipline. This is a certain road to victory.
~ from The Art of War, Giles translation ~

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