Chapter 2, Verses 2 and 3 - Sun Tzu

When you engage in actual fighting, if victory is long in coming, then men's weapons will grow dull and their ardor will be damped. If you lay siege to a town, you will exhaust your strength. Again, if the campaign is protracted, the resources of the State will not be equal to the strain.
~ from The Art of War, Giles translation ~

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