Because the Russians were kicking his ass, and he hoped Japan would help him out with the Russians.
If he thought that he was a fool.
The German Army was on the verge of taking Moscow when they declared war on us. So I don't think they were worried about Russia at that point.
So once again the puzzle of the 20th century is why did that nitwit declare war on the US when he didn't have to do so.
Looking at the all the evidence I think it was because he actually thought that Japan would win and he wanted his share of the spoils of a defeated US.
Greed.
It will get you every time.