The United States of Atlantis ~ Harry Turtledove

"Generals borrowed a lot of trouble. Any commander worth having needed to worry about how he'd respond if the enemy did this, that, or the other thing. Many of the things a general could come up with were wildly unlikely. MOST of the things a general could come up with never happened. But the day he didn't worry about them would be the day one came true."
The United States of Atlantis (Ch. 17) (Book 2 of the Atlantis series)