Warmatrix

An aircraft carrier is a ship that serves as launching and landing platform for aircraft. It acts like a mobile airfield at sea.
Quickly after the invention of airplanes, people tried to launch them from ships. As early ariplanes needed little speed to launch, a little space on board a normal ship sufficed. But later heavier aircraft required longer runways. The first true aircraft carrier, where planes could both take off and land, was the HMS Argus in 1918 CE. Like many first carriers, she was designed as something else and later converted into a carrier by placing a large flight deck on top of her.
Simple aircraft carriers have only a small flight deck, suitable only for Vertical-Take-Off-Landing (VTOL) aircraft, like helicopters. More advanced models have a short runway to allow fixed wing aircraft to gain some speed. These often curve up at the end in a 'ski' jump to give launching aircraft a final boost. They are used by Short-Take-Off-Vertical-Landing (STOVL) aircraft like jumpjets, or by conventional airplanes that are lightly loaded. The next category are the Short-Take-Off-But-Arrested-Recovery (STOBAR) carriers. They have longer runways and catch landing aircraft with nets. The largest, most powerful and expensive carriers are categorized as Catapult-Assisted-Take-Off-But-Arrested-Recovery (CATOBAR). These launch aircraft with steam-powered catapults and if necessary, upon landing catch them with a tail hook and arrestor wires. The most modern carriers have replaced these with electromagnetic catapults.
Carriers prefer to sail into the wind, so that the effective airspeed for the aircraft taking off and landing is greater than the sailing speed. Often the aircraft used on them are especially adapted to them: they can take off and land from the relatively short flight decks; some can fold their wings to take up less space when parked.
Aircraft carriers first saw serious action in World War II, especially on the Pacific ocean. The Japanese raid on Pearl Harbor that opened the war in that theater, was executed by bombers that were launched from carriers. Shortly afterwards heavy battleships proved vulnerable to attacks by torpedo bombers. They could destroy carriers if they came close, but the growing effectiveness of radar made that increasingly difficult and made carriers the stronger of the two. By 1942 CE, naval battles in the Pacific were dominated by aircraft carriers. Some of them were used as 'fleet carriers', mobile attack platforms, while others were classified as 'escort carriers', carrying fighter airplanes to protect fleets.
Before World War II, international naval treaties limited the size of capital ships, including aircraft carriers. As the war neared, several countries ignored the treaties and started to build large carriers, replacing battleships as the nuclei of fleets. The largest aircraft carriers currently in service are the American Nimitz class carriers, which entered service in the late 1960's CE. These are 330 meters long, fall in the CATOBAR category and can carry about 90 modern aircraft. They are nuclear powered and have crews of 5,000 people or more, making them a kind of floating towns.
Though the aircraft on board of carriers carry a lot of firepower, the ships themselves are vulnerable to counterattack. Losing a large and expensive aircraft carrier is a serious risk to a navy using them. Therefore carriers tend to be protected by a fleet of cruisers, destroyers and other auxiliary ships.