Tolland says that Hitler was told he didn't have to declare war against the US because the pact technically was a "non-aggression" treaty, and Japan had attacked the US. The US hadn't attacked Japan, which gave Germany a way out had it chosen it.

Hitler considered that, then decided the "honorable" thing to do was declare war against the US. (One of the few jokes Hitler appreciated prior to this was a gift from some of his military staff. They presented him with an empty box, and then said they were going to give him a plaque inscribed with the treaties he had kept. But there weren't any. Apparently Hitler thought that was funnier than hell.)

The US followed suit by declaring war against Germany.

And you're right. Diplomatic niceties aside, the Japanese knew they declared war with the Pearl Harbor attack. And so did the US.

Arf