Japanese attacks U.S
Plans for a surprise attack against the United States were begun as early as January of 1941.
Although it was Japanese Admiral Isoroku Yamamoto who initiated the plans for the attack against Pearl Harbor, Commander Minoru Genda was the plan's chief architect.
The Japanese used the codename "Operation Hawaii" for the attack on Pearl Harbor. This later changed to "Operation Z."
The Japanese specifically chose to attack on a Sunday because they believed Americans would be more relaxed and thus less alert on a weekend.