The simplest answer is men and terrain, but many things too numerous to mention did not change. For example, the spade (entrenching tool) has won more battles in world history than any weapon because it has not been rendered obsolete, and it does the same job today that it did in the Trojan War. Much WWI equipment was still used in WWII, but most countries abandoned the fixed defensive doctrine in favor of a mobile offensive doctrine. France and Belgium held onto their WWI doctrine of fixed fortifications, and their tanks still had one man doing both jobs of gunner and commander as in WWI, with disastrous results in WWII. The principles of aerial warfare remained unchanged as the zeppelin disappeared and the bomber and fighter improved. At sea, the battleship, cruiser, and destroyer remained the most important warships. The carrier had little chance to prove itself in WWI, and was still considered something of an experiment. The submarine, despite much success in WWI, was still considered a dirty and dishonarable weapon (the stiletto of the sea) because it attacked without warning and took no prisoners.
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World War 1 introduced aerial warfare with airplanes and trench warfare
more suffistication. atomic bombs contained more nucleus.
There was introduction of the atomic bomb, and mass production of tanks.
trench warfare chemical warfare
rules of warfare