Advances in technology and warfare

Aircraft were used for reconnaissance, as fighters, bombers and ground-support, and each role was advanced considerably. Innovation included airlift (the capability to quickly move limited high-priority supplies, equipment and personnel); and of strategic bombing (the bombing of civilian areas to destroy industry and morale). Anti-aircraft weaponry also advanced, including defences such as radar and surface-to-air artillery, such as the German 88 mm gun. The use of the jet aircraft was pioneered, and though late introduction meant it had little impact, it led to jets becoming standard in worldwide air forces.

U-995 Type VIIC at the German navy memorial at Laboe. Between 1939 and 1945, 3,500 Allied merchant ships (gross tonnage 14.5 million) were sunk at a cost of 783 German U-boats.

Advances were made in nearly every aspect of naval warfare, most notably with aircraft carriers and submarines. Although at the start of the war aeronautical warfare had relatively little success, actions at Taranto, Pearl Harbor, the South China Sea and the Coral Sea established the carrier as the dominant capital ship in place of the battleship. In the Atlantic, escort carriers proved to be 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 their not requiring to be as heavily armoured.Submarines, which had proved to be an effective weapon during the First World War were anticipated by all sides 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 theType VII submarine and wolfpack tactics. Gradually, improving Allied technologies such as the Leigh light, hedgehog, squid, and homing torpedoes proved victorious.

Land warfare changed from the static front lines of World War I 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 in increasing 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 utilised. 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 MG42, 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 postwar infantry weapon for most armed forces.

Most major belligerents attempted to solve the problems of complexity and security presented by using large codebooks for cryptography with the use of ciphering machines, the most well known being the GermanEnigma machine. SIGINT (signals intelligence) was the countering process of decryption, with the notable examples being the Allied breaking of Japanese naval codes and British Ultra, which was derived from methodology given to Britain by the Polish Cipher Bureau, which had been decoding Enigma for seven years before the war. Another aspect of military intelligence was the use of deception, which the Allies used to great effect, such as in operations Mincemeat and Bodyguard. Other technological and engineering feats achieved during, or as a result of, the war include the world's first programmablecomputers (Z3, Colossus, and ENIAC), guided missiles and modern rockets, the Manhattan Project's development of nuclear weapons, the development of artificial harbours and oil pipelines under the English Channel.