Actually, I think it was for shock value, as much as anything. He knew a long war was basically suicide for Japan. It's possible that he had a (very small) hope that a successful attack could shock the American people enough for them to sue for peace quickly. They were trying the same pattern as the previous Chinese and Russian conflicts that Japan won by grabbing what they wanted by surprise, then getting peace quickly. He thought he knew us, having been here as a Naval Attache, but evidently not well enough. I think that if he had known the US well enough to realize how violent the reaction by the average American would be, he wouldn't have attacked Pearl Harbor.