When his ship finally settled it was an hour before dawn, the safe hour, the time when most creatures, no matter what planet spawned them, are least alert. Or so his father had told him before he left Earth. Invading before dawn was part of the lore of Earth, hard-won knowledge directed solely toward survival on alien planets.
"But all this knowledge is fallible," his father had reminded him. "For it deals with that least predictable of entities, intelligent life." The old man had nodded sententiously as he made that statement.
"Remember, my boy," the old man went on, "you can outwit a meteor, predict an ice age, outguess a nova. But what, truthfully, can you know about those baffling and constantly changing entities who are possessed of intelligence?"
— from "Dawn Invader," in Store of the Worlds, by Robert Sheckley.