A carrier battle group (CVBG) consists of an aircraft carrier (designated CV) and its large number of escorts, together defining the group. The first naval task forces built around carriers appeared just prior to and during World War II. The Imperial Japanese Navy, IJN, was the first to assemble a large number of carriers into a single task force, known as Kido Butai. This task force was used with devastating effect in the Imperial Japanese Navy's Attack on Pearl Harbor. Kido Butai operated as the IJN's main carrier battle group until four of its carriers were sunk at the Battle of Midway. In contrast, the United States Navy deployed its large carriers in separate formations, with each carrier assigned its own cruiser and destroyer escorts. These single-carrier formations would often be paired or grouped together for certain assignments, most notably the Battle of the Coral Sea and Midway. By 1943 however, large numbers of fleet and light carriers became available, which required larger formations of three or four carriers. These groups eventually formed the Fast Carrier Task Force, which became the primary battle unit of the U.S. Fifth and Third Fleets.