World War II
Advances in technology and its application

Aircraft were used for reconnaissance, as fighters, bombers, and ground-support, and each role developed considerably. Innovations included airlift (the capability to quickly move limited high-priority supplies, equipment, and personnel); and strategic bombing (the bombing of enemy industrial and population centres to destroy the enemy's ability to wage war). Anti-aircraft weaponry also advanced, including defences such as radar and surface-to-air artillery, in particular the introduction of the proximity fuze. The use of the jet aircraft was pioneered and led to jets becoming standard in air forces worldwide.

Advances were made in nearly every aspect of naval warfare, most notably with aircraft carriers and submarines. Although aeronautical warfare had relatively little success at the start of the war, actions at Taranto, Pearl Harbor, and the Coral Sea established the carrier as the dominant capital ship (in place of the battleship). In the Atlantic, escort carriers became a vital part of Allied convoys, increasing the effective protection radius and helping to close the Mid-Atlantic gap. Carriers were also more economical than battleships due to the relatively low cost of aircraft and because they are not required to be as heavily armoured. Submarines, which had proved to be an effective weapon during the First World War, were expected by all combatants to be important in the second. The British focused development on anti-submarine weaponry and tactics, such as sonar and convoys, while Germany focused on improving its offensive capability, with designs such as the Type VII submarine and wolfpack tactics. Gradually, improving Allied technologies such as the Leigh Light, Hedgehog, Squid, and homing torpedoes proved effective against German submarines.

Land warfare changed from the static frontlines of trench warfare of World War I, which had relied on improved artillery that outmatched the speed of both infantry and cavalry, to increased mobility and combined arms. The tank, which had been used predominantly for infantry support in the First World War, had evolved into the primary weapon. In the late 1930s, tank design was considerably more advanced than it had been during World War I, and advances continued throughout the war with increases in speed, armour and firepower. At the start of the war, most commanders thought enemy tanks should be met by tanks with superior specifications. This idea was challenged by the poor performance of the relatively light early tank guns against armour, and German doctrine of avoiding tank-versus-tank combat. This, along with Germany's use of combined arms, were among the key elements of their highly successful blitzkrieg tactics across Poland and France. Many means of destroying tanks, including indirect artillery, anti-tank guns (both towed and self-propelled), mines, short-ranged infantry antitank weapons, and other tanks were used. Even with large-scale mechanisation, infantry remained the backbone of all forces, and throughout the war, most infantry were equipped similarly to World War I. The portable machine gun spread, a notable example being the German MG 34, and various submachine guns which were suited to close combat in urban and jungle settings. The assault rifle, a late war development incorporating many features of the rifle and submachine gun, became the standard post-war infantry weapon for most armed forces.

Most major belligerents attempted to solve the problems of complexity and security involved in using large codebooks for cryptography by designing ciphering machines, the most well-known being the German Enigma machine. Development of SIGINT (sig nals int elligence) and cryptanalysis enabled the countering process of decryption. Notable examples were the Allied decryption of Japanese naval codes and British Ultra, a pioneering method for decoding Enigma that benefited from information given to the United Kingdom by the Polish Cipher Bureau, which had been decoding early versions of Enigma before the war. Another component of military intelligence was deception, which the Allies used to great effect in operations such as Mincemeat and Bodyguard.

Other technological and engineering feats achieved during, or as a result of, the war include the world's first programmable computers (Z3, Colossus, and ENIAC), guided missiles and modern rockets, the Manhattan Project 's development of nuclear weapons, operations research, the development of artificial harbours, and oil pipelines under the English Channel. Penicillin was first developed, mass-produced, and used during the war.
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