In the early 1920s, the Washington Naval Treaty imposed limits on the maximum size and total tonnage of aircraft carriers for the five main naval powers. Later treaties largely kept these provisions. As a result, construction between the World Wars had been insufficient to meet operational needs for aircraft carriers as World War II expanded from Europe. Too few fleet carriers were available to simultaneously transport aircraft to distant bases, support amphibious invasions, offer carrier landing training for replacement pilots, conduct anti-submarine patrols, and provide defensive air cover for deployed battleships and cruisers. The foregoing mission requirements limited use of fleet carriers unique offensive strike capability demonstrated at the Battle of Taranto and the Attack on Pearl Harbor. Conversion of existing ships (and hulls under construction for other purposes) provided additional aircraft carriers until new construction became available.
Conversions of cruisers and passenger liners with speed similar to fleet carriers were identified by the U.S. as "light aircraft carriers" (hull classification symbol CVL) able to operate at battle fleet speeds. Slower conversions were classified as "escort carriers" and were considered naval auxiliaries suitable for pilot training and transport of aircraft to distant bases.
The Royal Navy had recognized a need for carriers to defend its trade routes in the 1930s. While designs had been prepared for "trade protection carriers" and five suitable liners identified for conversion, nothing further was done mostly because there were insufficient aircraft for even the fleet carriers under construction at the time. However, by 1940 the need had become urgent and HMS Audacity was converted from the captured German merchant ship MV Hannover and commissioned in July 1941. For defense from German aircraft, convoys were supplied first with fighter catapult ships and CAM ships that could carry a single (disposable) fighter. In the interim, before escort carriers could be supplied, they also brought in merchant aircraft carriers that could operate four aircraft.
In 1940, Admiral William Halsey recommended construction of naval auxiliaries for pilot training. In early 1941 the British asked the US to build on their behalf six carriers of an improved Audacity design but the US had already begun their own escort carrier. On 1 February 1941, the United States Chief of Naval Operations gave priority to construction of naval auxiliaries for aircraft transport. U.S. ships built to meet these needs were initially referred to as auxiliary aircraft escort vessels (AVG) in February 1942 and then auxiliary aircraft carrier (ACV) on 5 August 1942. The first U.S. example of the type was USS Long Island. Operation Torch and North Atlantic anti-submarine warfare proved these ships capable aircraft carriers for ship formations moving at the speed of trade or amphibious invasion convoys. U.S. classification revision to escort aircraft carrier (CVE) on 15 July 1943 reflected upgraded status from auxiliary to combatant. They were informally known as "Jeep carriers" or "baby flattops". It was quickly found that the escort carriers had better performance than light carriers, which tended to pitch badly in moderate to high seas. The Commencement Bay class was designed to incorporate the best features of American CVLs on a more stable hull with a less expensive propulsion system.
Among their crews, CVE was sarcastically said to stand for "Combustible, Vulnerable, and Expendable". Magazine protection was minimal in comparison to fleet aircraft carriers. HMS Avenger was sunk within minutes by a single torpedo, and HMS Dasher exploded from undetermined causes with very heavy loss of life. Three escort carriersUSS St. Lo, Ommaney Bay and Bismarck Seawere destroyed by kamikazes, the largest ships to meet such a fate.
Allied escort carriers were typically around 500 ft (150 m) long, not much more than half the length of the almost 900 ft (270 m) fleet carriers of the same era, but were less than 1/3 of the weight. A typical escort carrier displaced about 8,000 long tons (8,100 t), as compared to almost 30,000 long tons (30,000 t) for a full-size fleet carrier. The aircraft hangar typically ran only 1/3 of the way under the flight deck and housed a combination of 2430 fighters and bombers organized into one single "composite squadron". By comparison, a late Essex-class fleet carrier could carry a total of 103 aircraft organized into separate fighter, bomber and torpedo-bomber squadrons.
The island on these ships was small and cramped, and located well forward of the funnels (unlike on a normal-sized carrier where the funnels were integrated into the island). Although the first escort carriers had only one aircraft elevator, having two elevators (one fore and one aft), along with the single aircraft catapult, quickly became standard. The carriers employed the same system of arresting cables and tail hooks as on the big carriers, and procedures for launch and recovery were the same as well.
The crew size was less than 13 of that of a large carrier, but this was still a bigger complement than most naval vessels. It was large enough to justify the existence of facilities such as a permanent canteen or snack bar, called a gedunk bar, in addition to the mess. The bar was open for longer hours than the mess and sold several flavors of ice cream, along with cigarettes and other consumables. There were also several vending machines available on board.
In all, 130 Allied escort carriers were launched or converted during the war. Of these, six were British conversions of merchant ships: HMS Audacity, Nairana, Campania, Activity, Pretoria Castle and Vindex. The remaining escort carriers were U.S.-built. Like the British, the first U.S. escort carriers were converted merchant vessels (or in the Sangamon class, converted military oilers). The Bogue-class carriers were based on the hull of the Type C3 cargo ship. The last 69 escort carriers of the Casablanca and Commencement Bay classes were purpose-designed and purpose-built carriers drawing on the experience gained with the previous classes.