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.
I believe we had “shoot on sight” for U-boats about six months before Pearl Harbor. So we were already technically at war against Germany.
“I always wondered had he not declared war on the US, what would have happened.”
There’s no doubt. Once we were in we were in. They could have pointed to some American tourist who got stuck in Poland and didn’t even die but was moderately inconvenienced for justification. To be in the war was to be in the war. There would never have been a seperate war for Japan.
I think the reverse is true.
In the fall of 1941 there was a great debate amongst Japanese leaders as to whether they should support Hitler and attack Russia (favored by the Japanese Army) or leave Hitler to his own resources and turn south toward the US, British and Dutch colonies for their natural resources, especially oil (favored by the Navy).
The Japanese Navy not only won the debate, but had the "Russia First" advocates purged from their positions of influence.
How could this happen? I think because Hitler himself did not oppose it. And that's because, by the fall of 1941 Hitler had been on a two year military roll -- rolling over every obstacle put in his way. The Russians were just pushovers -- he had already killer or captured millions more Russian soldiers than the numbers of Germans sent on Operation Barbarosa.
Point is: Hitler did not then believe he would need the Japanese to help him out around Moscow. So he told them, in essence: go ahead and attack the Americans.
Turned out, the Russians were not quite as weak as Hitler's experience had lead him to believe -- especially Stalin's best troops, which in 1939 had already fought and defeated the best the Japanese had to offer.