Some people believed that because of the Japanese attacked the USS Panay which was sent to China to sail up the Yungtze River in 1937 to rescue Americans after a battle between Chinese Forces and Japan, there shouldn't have been any doubts. The Japanese claimed it was mistake, but the U.S. said the ship was covered in American flags.
The commander at Pearl Harbor testified at the Pearl Harbor hearings that he was purposely being confused by Washington in terms of orders etc. etc. Other folks point to the economic war FDR had launched against Japan, and that he had to know that would ultimately lead to war.