“One of the mysteries of WWII is why Hitler declared war on the US after Pearl Harbor when he didn’t have to. “
In the sense that no one ever has to do anything yeah. But Hitler had entered into a mutual defense pact with the Japanese so that the Japanese would keep a bunch of British and Russian forces busy in Asia.
And besides, Hitler was already in what amounted to an undeclared shooting war with the US with all the US ships that his u-boats were sinking. Plus he (correctly) considered the US to be pathetically underarmed . Plus he (mistakenly) thought that he would be finished with Russia before the US could be a factor. Plus he (probably mistakenly) believed that once he finished of Russia that the US and British would lose interest in the war.
I’ve never understood why anyone considered that decision a mystery. It made perfect sense based on Hitler’s assumptions.