(from ebookjapan):

In 1941, a secret Japan-Germany summit meeting was held in Mexico, attended by Isoroku Yamamoto from Japan and Hitler from Germany. Fighting in Africa, the Middle East, the Mediterranean, the Atlantic, and against the mighty Russia, and with the U.S. entry into the war approaching, Hitler asked Japan for cooperation.

The Imperial Japanese Navy organizes a special paratroop unit called "Raijin" to launch an invasion of the U.S. mainland...!