In Europe, before the outbreak of the war, the Allies had significant advantages in both population and economics. In 1938, the Western Allies (United Kingdom, France, Poland and British Dominions) had a 30 percent larger population and a 30 percent higher gross domestic product than the European Axis (Germany and Italy); if colonies are included, it then gives the Allies more than a 5:1 advantage in population and nearly 2:1 advantage in GDP. In Asia at the same time, China had roughly six times the population of Japan, but only an 89 percent higher GDP; this is reduced to three times the population and only a 38 percent higher GDP if Japanese colonies are included.
Though the Allies' economic and population advantages were largely mitigated during the initial rapid blitzkrieg attacks of Germany and Japan, they became the decisive factor by 1942, after the United States and Soviet Union joined the Allies, as the war largely settled into one of attrition. While the Allies' ability to out-produce the Axis is often attributed to the Allies having more access to natural resources, other factors, such as Germany and Japan's reluctance to employ women in the labour force, Allied strategic bombing, and Germany's late shift to a war economy contributed significantly. Additionally, neither Germany nor Japan planned to fight a protracted war, and were not equipped to do so. To improve their production, Germany and Japan used millions of slave labourers; Germany used about 12 million people, mostly from Eastern Europe, while Japan used more than 18 million people in Far East Asia.
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 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, 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 air forces worldwide.
Advances were made in nearly every aspect of naval warfare, most notably with aircraft carriers and submarines. Although aeronauticalwarfare 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 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 the Type 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 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 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 MG34, 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 involved in using large codebooks for cryptography by designing ciphering machines, the most well known being the German Enigma machine. Development of SIGINT 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 benefiting from information given to Britain by thePolish Cipher Bureau, which had been decoding early versions of Enigma 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 programmable computers (Z3, Colossus, and ENIAC), guided missiles and modern rockets, the Manhattan Project's development of nuclear weapons, operations research and the development of artificial harbours and oil pipelines under the English Channel.