To some degree the atom bombs dropped on Hiroshima and Nagasaki which brought about the surrender of the Empire of Japan in August 1945, seemed to validate this, at least in postwar propaganda.
After World War I many innovations in metals and chemicals had created withering firepower that made defenses effective. In France the lesson was translated into the Maginot Line which was supposed to hold a line at the border with Germany. France had also more, and many better, tanks than Germany as of the outbreak of their hostilities in 1940. As in World War I, the French generals expected that armour would mostly serve to help infantry break the static trench lines and storm machine gun nests. They thus spread the armour among their infantry divisions, ignoring the new German doctrine of blitzkrieg which called for massed armour attacks (against which there was no effective defense but mobile anti-tank guns - infantry anti-tank weapons having not been invented yet).
Air power was a major concern of Germany and Britain between the wars. Amazingly, however, trade in aircraft engines continued, with Britain selling hundreds of its best to German firms - which used them in a first generation of aircraft, and then improved on them much for use in German aircraft.
Depending on one's frame of reference, one can reasonably assert that World War II began with the Japanese invasion of Manchuria in 1931, or as late as the last declarations of war between the United States and Germany in December 1941. Quite a bit occurred during this time to escalate technological conflict, most notably the upgrading and deployment of aircraft carriers by the U.S. and Japan in the Pacific, and invention of carrier-type aircraft such as the Mitsubishi Zero, largely considered the best plane of its time.
more on aircraft carriers and 1930s innovations in military technologies
(Radio, radar, aerial photography)
(Shipping, submarines, bombing, the draft, civilian labour in Germany vs. USA)
(Fighters vs. bombers), carrier battles and air attacks on ships in the Pacific - diminished role of the battleship.
(Island hopping, seaborne invasions Dieppe, Sicily and Normandy)
(Mass tank battles, e.g. Kursk)
Extreme mobility (gliders, paratroops, bicycles, boats, landing craft)
Horrifying city battles (Stalingrad, Berlin) and seiges (Leningrad, London) from ground and air.
See also:
Between the wars
The beginnings
Command and control
War of attrition
Air warfare
Beaches
Tanks
Mobility
Urban warfare
Sources