I always thought that Japan and Germany had a mutual defense treaty which required either one to declare war if any country declared war on either.
Japan initiated hostilities, Germany was not obliged to bail them out. In the event, Hitler might feared that the U.S. would finish off Japan and then turn on Germany. Official American policy at the start of the war was to devote 80% of our resources to Europe and 20% to the Pacific. After Midway, the Pacific got more resources, but it was still largely a side show from the bigger war. If the U.S. only had to take on Japan, we would really have steamrollered them, and been left with a big well oiled military machine and an unsinkable aircraft carrier, HMS Great Britain.
It's more than just that, though. The U.S. was effectively at war with Germany long before any formal declarations were made anyway. The U.S. was shipping arms to Great Britain across the North Atlantic under U.S. Navy escort.
I've never understood this idiotic infatuation with war declarations or even open military attacks like Pearl Harbor as a benchmark for determining when a war "officially" began.
The Pact presented the condition "ARTICLE 3....to assist one another with all political, economic and military means when one of the three contracting powers is attacked..." by a country not already involved in the war,"
Technically though, the US and the allies didn't attack Japan so Germany was not obliged to declare war on the US based on the pact.
No, mutual assistance agreement.
German generals were shocked Hitler declared war on America. His greatest folly and there were a lot of them but that one took the cake.