in this action. Ultimately, the maturity of air power spelled doom for the battleship. Battleships in the Pacific ended up primarily performing shore bombardment and anti-aircraft defense for the carriers. Only the fast battleships (formerly battlecruisers) of the Kongo class saw much action due to their speed, while the slower and heavier battleships were held in reserve for a decisive engagement of battleships versus battleships which never really happened. Yamato and were sunk by air attacks long before coming in gun range of the American fleet. Aircraft carriers In the 1920s, the (originally laid down as a battleship) and a similar ship, the (originally laid down as a battlecruiser) were converted to aircraft carriers (航空母艦 Kōkūbokan) to satisfy the terms of the Washington Naval Treaty. From 1935–1938, Akagi and Kaga received extensive rebuilds to improve their aircraft handling capacity. Japan put particular emphasis on aircraft carriers. The Imperial Japanese Navy started the Pacific War with 10 aircraft carriers, the largest and most modern carrier fleet in the world at that time. There were eight American aircraft carriers at the beginning of hostilities, only three operating in the Pacific; and eight British aircraft carriers, of which a single one operated in the Indian Ocean. A large number of these Japanese carriers were of small size, however, in accordance with the limitations placed upon the Navy by the London and Washington Naval Conferences. Nonetheless the Japanese initially had the upper hand over the American and British, by grouping all of their fleet carriers into a single unit known as the 1st Air Fleet or Kidō Butai ("Mobile Force"). In the Kidō Butai, the two -class carriers were superior to any carrier in the world, until the wartime appearance of the American Essex class. Following the Battle of Midway, in which four Japanese fleet carriers were sunk, the IJN suddenly found itself short of fleet carriers (as well as trained aircrews), robbing them of a strategic offensive capability. The IJN consequently undertook an ambitious set of projects to convert commercial and military vessels into carriers, such as the . Another conversion project, , was based on an incomplete Yamato-class super battleship and became the largest-displacement carrier of World War II. One exception was the , which was the only Japanese carrier with an armored flight deck and first to incorporate a closed hurricane bow. All three mid-war designs were sunk in 1944, with