The Japanese planned to use a new type of warfare
In November 1940, Britain’s Royal Navy launched a carrier-based air raid against the main anchorage of the Italian fleet in Taranto. It was the first such strike in history, and it successfully damaged or destroyed several of Italy’s most powerful warships as they lay at anchor. The US Navy noted the success, though outside of its aviation wing the attack was largely ignored. The Japanese took a far deeper interest. The British had destroyed Italian battleships, sinking them at their moorings, using just 21 obsolescent torpedo bombers from one aircraft carrier. The harbor in which the attack took place was, like the American fleet anchorage at Pearl Harbor, relatively shallow, and protected with anti-aircraft batteries and supporting fighter aircraft. The level of success achieved by the British intrigued the Japanese planners. They proposed a similar strike, though with far more aircraft and carriers.
At the time, standard naval doctrine for aircraft carriers in all of the world’s navies were they operated in small task groups, serving the main fleets as scouts. Capable of hit and run raids, they were not viewed as lethal against capital ships. Japanese planners, led by Commander Minoru Genda, proposed massing carriers together, combining their aircraft in a coordinated strike. The Japanese plan was in essence a seaborne blitzkrieg, in which enemy air would be destroyed on the ground before additional waves of attacks demolished shipping at anchor, docks and shore facilities, and most critically, the American aircraft carriers and battleships before they could respond. Several technical issues presented difficulties to the plan. For one, the shallowness of Pearl Harbor meant aerial torpedoes would plunge to the bottom upon release from their host aircraft. Another was the lack of an aerial bomb capable of penetrating battleship armor.