If there even were any doubts by Hitler that he would have to fight the United States, the last shreds of it were cast aside on December 4th 1941 when the Chicago Tribune leaked the Rainbow 5 War Plan. In the article it specifically lays out the “Germany First” plan to defeating the Axis powers, with an emphasis on containing Japan until this is accomplished. This leak would likely have grown into a scandal had the Japanese not attack a mere 3 days later.
Hitler though saw what the plan said and I feel that is what prompted him to declare war on the 11th. This was a great error on his part because he was under no obligation to do so and had he not, the United States probably could not have declared war on Germany. The isolationists were still strong enough, despite the Japanese attack to for FDR to declare war on Japan only. It is one of the reasons there is no mention of Germany in his Day of Infamy speech. If FDR felt he could tie all of this together, I’m certain there would have been at least some reference to “Axis” powers in that speech.
Based on his March 1940 propaganda, and other points mentioned by posters today, I think in Hitler's mind, the US was already at war with Axis powers in all but name.
And what did that name matter anyway? Can you name any other countries that Hitler officially declared war on?
So why make a big deal of declaring war on the US?
Answers:
A) Moral support for his Japanese allies, and
B) to release his U-boats for unrestricted sinkings along the US coastline.
But if Hitler declaring war on the US in December 1941 was a "mistake," was it not of a kind with invading the Soviet Union that June?
Both in hindsight seem reckless and premature.
Would he not have been better off to wait a year, just as Stalin expected he would?
Instead, Hitler's mindset in 1941 concluded that time was not on his side -- so he must invade now, declare war now, because a year later the forces against Germany would no longer be as weak.
Is that true?
Would Hitler have been better off to wait a year before invading Russia and declaring war on the US?
Answer:
Yes, possibly, but if and only if Hitler had been willing to do in 1940 and '41 what he ended up doing in 1943 & '44 -- appoint an Albert Speer to bring in all of conquered & axis Europe as suppliers to the German war industry.
But such a move was inconceivable ideologically, and only attempted after Hitler's dreams of quick-easy victories had thoroughly evaporated.