The Imperial Japanese Navy (日本海軍, Nihon Kaigun) at the beginning of World War II contained the world's largest aircraft carrier fleet. At the centre, was the 1st Air Fleet (第一航空艦隊, Dai-ichi KōKū Kantai) which was a grouping of naval aircraft and aircraft carriers. With the Kido Butai (being its primary carrier battle group) the Imperial Japanese Navy would reign supreme for the first six months after Japan's entry to World War II.