The first naval fleets built around carriers were seen at the beginning of World War II. The Imperial Japanese Navy (IJN) was the first to put many carriers into one task force. This task force was used in the attack on Pearl Harbor. It was the IJN's main carrier battle group until four of its carriers were sunk at the Battle of Midway. The United States Navy deployed its carriers in separate formations. Each carrier was put with its own cruiser and destroyer escorts. These single-carrier fleets would often work together for certain assignments. The most notable examples of this are the Battle of the Coral Sea and Midway. By 1943, there was a large number of fleet and smaller carriers became available. This led to them using larger formations of three or four carriers. These groups became the Fast Carrier Task Force. They are the primary battle unit of the U.S. Third and Fifth Fleets.