Yeah, Germany declared war on the US on Dec 10 (I think). Which still surprises people today. Japan and Germany weren’t all that close despite the treaty they had. So I’m surprised Germany thought the Japanese would declare war on Russia if Germany declared war on the US.
Hitler at first doubted it as well, but he got reassurances from Japan that they would help against Russia, then afterwards, he agreed to declare war on the US.
I always wondered had he not declared war on the US, what would have happened. My guess is the US would have just gone ahead and beefed up arms shipments to Britain knowing eventually the Germans would attack one of our ships, this giving us Casus Belli.