Kaga was an aircraft carrier built for the Imperial Japanese Navy (IJN) and was named after the former Kaga Province in present-day Ishikawa Prefecture. Originally intended to be one of two Tosa-class battleships, Kaga was converted under the terms of the Washington Naval Treaty to an aircraft carrier as the replacement for the battlecruiser Amagi, which had been irreparably damaged during the 1923 Great Kanto earthquake. Kaga was rebuilt in 1933-1935, increasing her top speed, improving her exhaust systems, and adapting her flight decks to more modern, heavier aircraft.
The ship figured prominently in the development of the IJN's carrier striking force doctrine, which grouped carriers together to give greater mass and concentration to their air power. A revolutionary strategic concept at the time, the employment of the doctrine was crucial in enabling Japan to attain its initial strategic goals during the first six months of the Pacific War.
(More on Wiki HERE)