Why Germany was always the underdog in World War II

It may be that they were less equipped but were it not for a number of mistakes by Hitler and our joining the Allies, a good section of Europe would be German speaking today.

Despite a decade of warnings, the Nazis achieved strategic surprise repeatedly - In what was left of Czechoslovakia after the Sudatenland was amputated, in Poland, in the Low Countries, in Norway (a really ballsy operation when you read about it), In France, and in the USSR. They were able to parlay that surprise into tremendous advantage and gains, but at key points screwed up and allowed the Allies to hang on.
I would argue that Germany had a legitimate chance to win, were it not for Dunkirk, El Alamein, Stalingrad, or Kursk. I think if any two of those had gone the other way, one or more of the Allied powers would have been knocked out of the war.

Japan, by contrast never had the resources for a protracted war with the USA and tried for a knockout in the first round that didn't achieve the knockout. They damn near achieved it against the UK by forcing the disaster at Singapore, but not quite. After 1942 it was only a matter of time. Midway was their Waterloo.