I think it ought to go without saying that this means the WWII era.
Ahem. Throughout the late 1930s, Japan made a string of aggressive moves towards its neighbors, including the Soviet Union. What made the Russians unusual was that Japan's little advances were beaten handily. This eventually convinced the Japanese to avoid confrontation in the North as they became committed in China and, eventually, the rest of Asia.
Let's suppose [
waves hand] that for one reason or another, the Europeans look too strong to tackle, even from a Japanese military perspective. That means the Japanese aren't in Indochina, and conversely that they aren't - for the moment - under American embargo. Let's also say [
hand waves vigorously] that the Soviet Union is at war in the west. Not in a life-or-death struggle like Barbarossa, but at war.
The Japanese are grappling with as much of China as they can reach, just as in OTL. It is very difficult, and there is a lot of pressure to find quick-fixes to make the job easier. In OTL, that meant driving the Europeans out of East Asia, but in this one that doesn't look like an option. And the Soviets
are actively offering support to the Chinese. Could the Japanese convince themselves they could take on the Soviets, if only they yell "banzai" loud enough?