I am confused, why would it only be Germany (30000 soldiers in 400 000 is insignificant) who'd fight against USSR if the Axis was supposedly declaring war to fight TOGETHER. (I have little to no knowledge on WWII so please go easy on me)
Japanese and German collaboration was much less extensive than allied cooperation. Great geographic distances and differing objectives made their alliance very tenuous.
u/kieslowskifan has a typically excellent answer that might help you out [here.] (https://www.reddit.com/r/AskHistorians/comments/aemmvb/why_japan_didnt_join_with_germany_during_first/)