Let’s start with why Japan didn’t declare war on the Soviet Union. I left a comment a while back here that explains the events of why Japan never attacked the Soviet Union after their border conflict. But to summarize it, the Japanese army tested the Soviet forces in Khalkin Gol and lost to their forces. And after this undeclared border conflict Japan realized it could not stand up to Soviet land forces. So in 1941 they signed a neutrality pact with each other, with this neither side would declare war on the other. However Japanese delegates and ambassadors assured Hitler and Germany during Operation Barbarossa that Japan would attack Russia any day now. But the truth was that Japan was waiting for the Soviets to be completely broken before entering this war.

Germany’s and to a lesser extent Italy’s declaration of War on the US was more or less an attempt to keep them occupied with Japan. Hitler knew he was going to fight the US and they were going to be a problem down the line. To a point the US were already a problem by 1941. With the Destroyer for Bases act, Lend Lease, and US navy escorts of convoys they were helping the UK and USSR and were part of the Allies. While some in Kriegsmarine just wanted to sink US ships, Hitler tried to avoid this as he was not ready yet to fight the US. But his plans of a quick victory against Russia was dashed by November of 1941 as they didn’t collapse as he predicted. Hitler also suspected the US would join in the war soon, either by the end of this year or by 1942. While many claim he didn’t acknowledge the USA’s power, this is false. He acknowledged in his Second Book that the US was a great threat and its industry would be a problem.

So he made closer ties to Japan as they had a large navy and could tie down the US. His hopes were that while Japan either defeated the US or kept them tied down for some time, they could not interfere with his plans of invading and conquering Russia. It was also hoped that Japan would also help in the invasion of Russia by attacking from the East. After this was done, Hitler could focus on dealing with the US and UK. He also encouraged the Japanese to attack the US and its territories in the Pacific and in return Germany would back Japan and help them. After the attack on Pearl Harbor, Hitler was immediately impressed by the Japanese attack and swept by this and some urging by Ribbentrop his foreign minister he declared war on the US. But he was shocked that it happened all of a sudden and it took a few days for him to declare war on the US or attack US ships.

While Japan didn’t declare war on Russia during Barbarossa; Germany hoped by declaring war on the US would bring them and the Japanese closer together. And in turn time down the US while they finished their conquest of Russia and Europe. But what happened in the end was Japan’s once great fleet at the bottom of the ocean, and Germany fighting three world powers on two fronts.

Sources:

Bosworth, Richard, and Joseph Maiolo. The Cambridge History of the Second World War. Volume 2: Politics and Ideology. Cambridge University Press, 2015.

Hitler, Adolf, and Salvator Attanasio. Hitlers Secret Book: Zweites Buch: the Unpublished Sequel to Mein Kamph. CreateSpace Independent Publishing Platform, 2017.

Wegner, Bernd (research Institute Of Military History, Pots, and Derry Cook-radmore. Germany and the Second World War - Volume Vi: the Global War. Oxford University Press, 2015