In April 1941, Japan assembled its Kidō Butai ("Mobile Force"), the most powerful naval unit that had ever existed. It was the culmination of fourteen years of experimentation and refinement, and had become the core of Japan's national defence capability. Also known as the First Air Fleet, it contained all four of Japan's fleet carriers and all three of Japan's light carriers along with 474 aircraft, supporting battleships, cruisers, and destroyers. This innovative concentration of aircraf…