The Japanese had been impressed with Admiral Andrew Cunningham's Operation Judgement (the Battle of Taranto), where 20 elderly biplanes (Fairey Swordfish) launched from a carrier force way in advance of the main British base at Alexandria disabled half the Italian battle fleet and forced the withdrawal of the Italian fleet to behind Naples. Admiral Isoroku Yamamoto dispatched a naval delegation to Italy, which concluded that a larger and better-supported version of Cunningham's brilliant maneuver could force the U.S. fleet back to California, giving time to achieve the "Greater East Asia Co-Prosperity Sphere"- shorthand for control of the oil reserves of the Dutch East Indies, with a defensible buffer around them. Most importantly, the delegation returned to Japan with the secret of the shallow running torpedo which Cunningham's "boffins" had devised.

Additionally, some Japanese strategists may have been influenced by the actions of U.S. Admiral Harry Yarnell in the 1932 joint Army-Navy exercises, which assumed an invasion of Hawaii. Yarnell, playing the role of Commander of the attacking fleet, sailed his aircraft carriers northwest of Oahu in rough weather, and launched 'attack' planes on the morning of Sunday, 7 February 1932. Judges assigned to the exercise noted that Yarnell's aircraft were able to inflict serious 'damage' on the defenders, who were unable to locate his fleet for 24 hours after the attack. Conventional US Navy doctrine of the time (and other naval opinion as well) believed that any attacking force would be set upon and destroyed by the battleship fleet stationed at Pearl Harbor, and dismissed Yarnell's strategy as impractical in the real world.

Yamamoto began considering such an attack early in 1941, and after some pressure on Naval Headquarters (including a threat to resign), managed to get permission to begin formal planning and training. The events of the summer (see above) led to preliminary approval of the attack plan at an Imperial Conference (which included the Emperor) and then approval of the attack in another Imperial Conference early in November.

The Japanese fleet steamed towards Pearl Harbor undetected until the last moment. Or so they say.

The intent of the attack on Pearl Harbor was to neutralize American naval power in the Pacific, if only temporarily, as part of a theater-wide, near-simultaneous coordinated attack against several different countries. Yamamoto himself expected that even a successful attack would gain only a year or so of freedom of action before the U.S. fleet recovered enough to check Japanese advances. Preliminary planning for a Pearl Harbor attack in support of military advance elsewhere began in January 1941, and, after some Imperial Navy factional infighting, the project was finally judged worthwhile. Training for the mission was under way by mid-year. The planned attack depended primarily on torpedoes, but the weapons of the time required deep water to function if air launched. This was a critical problem because Pearl Harbor is shallow, except in dredged channels. Over the summer of 1941, Japan secretly created and tested torpedo modifications that could be expected to work properly in a shallow water drop. The effort resulted in the Type 95 torpedo which inflicted most of the damage to U.S. ships during the attack. Japanese weapons technicians also produced special armor-piercing bombs by fitting fins on 14 and 15 inch (356 and 381mm) naval gun shells. These were able to penetrate the armored decks of battleships and cruisers when dropped from 10,000 feet (3,000m), if they could actually hit them.

On November 26, 1941, a fleet including six aircraft carriers commanded by Japanese Vice Admiral Chuichi Nagumo left Hitokappu Bay in the Kuril Islands under orders for strict radio silence bound for Hawaii. The aircraft carriers involved in the attack were: Akagi, Hiryu, Kaga, Shokaku, Soryu, and Zuikaku. Two fast battleships, 2 heavy cruisers, 1 light cruiser, 9 destroyers, and 3 fleet submarines provided escort for the task force. The carriers had a total of 423 planes, including Mitsubishi Type 0 "Zero" fighters, Nakajima Type 97 "Kate" torpedo bombers, and Aichi Type 99 "Val" dive bombers. The Japanese task force and its air group were larger than any previous aircraft carrier based strike force. Accompanying the fleet were 8 tankers for refueling. In addition, the Advanced Expeditionary Force included 20 fleet submarines and 52-man Ko-hyoteki-class midget submarines; they were to gather intelligence and sink any U.S. vessels that might try to flee Pearl Harbor during the air attack.