Chapter 10, Verses 4 and 5 - Sun Tzu

Ground which can be abandoned but is hard to re-occupy is called entangling. From a position of this sort, if the enemy is unprepared, you may sally forth and defeat him. But if the enemy is prepared for your coming, and you fail to defeat him, then, return being impossible, disaster will ensue.
~ from The Art of War, Giles translation ~

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