This is ahistorical, while the Soviet Union wasn't at war with Japan until the late stages they provided critical support to the Chinese Communists and Nationalists who had been fighting the Japanese since 1937 and forced the creation of the Second Unified Front against Japan (the Nationalist-Communist coalition that eventually drove Japan out of China). The Nationalists were so against uniting with those dirty Commies against Japan that Chiang had to be physically kidnapped and kept on house arrest until he would agree to form the Second Front!
There's a lot you can criticize the Soviet Union for, but their fight against fascism during the first half of the 20th century was not one of them.
This game hard as fuck