And Britain declared war on Germany. The German people were also unenthusiastic about WWII.

Also the U.S. was de facto in the war already thanks to its supply arrangements with both Britain and Russia. If they had wanted to stay out of the war they probably could have.

You are correct though. Hitler's declaration of war on the U.S. always seemed incredibly reckless to me. I am not a historian but a recent book suggested that he was more concerned with American money power (which he saw as Jewish) than conventional historians had thought previously.

DW:

Brendan Simms summarizes his main thesis: Hitler's driving force in domestic and foreign policy was born out of a love-hate relationship with "Anglo-America." It was not the fear of communism and the Soviet Union that led him to war and destruction, but rather the struggle with Great Britain and the United States and the fear of international capitalism.

[Snip...]

According to Simms, even Hitler's anti-Semitism did not arise primarily from a deep hatred of Jews, but secondarily, from a competition with "world capitalism" based in the US, where Jews were sitting in positions of power. [Cont...]

The book is controversial amongst court historians — Evans politely says it is dishonest rubbish— possibly because it accords with some of the views of the alt-right. It also, given the social turmoil in Germany during the 1920s, seems ridiculous Hitler wasn't concerned with Soviet power.

My own view is that history is open to so many interpretations, especially when it comes to matters of emphasis and nuance that it becomes a form of myth making — a means by which we interpret our current circumstances despite it being rooted in objective fact.