It's LN vol.3 Chapter 3: Director Maefuji's Unsolved Case files. p. 126 -127.
Edit: I got one thing wrong though. It's not an act of revenge. The Soviet already wanted to take Sakhalin back.
There's an explanation about this in LN. If I remember correctly, Japan got to keep Sakhalin at the time of surrender but lost it during the Manchurian War. Basically, The Soviet invaded Manchuria but was pushed back by US-Japan coalition. As an act of revenge, the Soviet invaded Sakhalin. By...