Steven Den Beste is discussing the various alliances of WWII and how they influenced the war here.
He notes that Germany and Japan were trying to exploit each other in various ways, claiming that Hitler declared war on the US after Pearl Harbor because he thought he had a deal that the Japanese would attack the Russians in the East. That would tie up Russian troops in Asia, lightening the load on the Wehrmacht.
The Japanese never came through, but the Russians couldn't be sure of that at first, so they kept forces in Asia in case the Japanese attacked.
Then, through the work of a German Communist spy named Richard Sorge, they discovered that the Japanese had no plans to attack in the East. As a result the Russians would redeploy troops stationed in Asia to fight the Germans.
Sorge operated out of the German embassy in Japan, so he had access to incredibly valuable information, and he proved his value to the Russians again and again. Through him the Russians had warning of many significant events, including the German invasion of Russia.
Sorge was caught by the Japanese and hanged in 1944.