Chapter 12, Verses 21 and 22 - Sun Tzu

But a kingdom that has once been destroyed can never come again into being; nor can the dead ever be brought back to life. Hence the enlightened ruler is heedful, and the good general full of caution. This is the way to keep a country at peace and an army intact.
~ from The Art of War, Giles translation ~

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