Failure to understand the United States industrial capacity
Both the Germans and the Japanese failed to grasp the industrial power of the United States, and its ability to arm, feed, and clothe its own troops and those of its Allies, as well as replace lost ships to carry the materiel to them. During the period between December 1941 and August 1945 the United States built the largest navy in the history of the world, the largest merchant marine in the history of the world, and more airplanes than any other country. In 1944 the United States produced more airplanes than Japan did in all of the war years combined. Although Yamamoto never made the often quoted line that he had awakened a sleeping giant, the sentiment was nonetheless true.
Hitler was contemptuous of the United States, believing its people to be soft and spoiled, distracted by living well and avoiding commitment and hard work. He did not believe that the Americans could launch the war effort which would be necessary to overthrow his regime. Hitler also personally hated Franklin Roosevelt, considering him to be the epitome of the idle rich. The population of the United States was, in Hitler’s estimation, mongrelized, unaware of world affairs, and not likely to give up their creature comforts to support a war in Europe. The massive American industrial effort in the Second World War was simply beyond the comprehension of the Axis powers.