Here are some past posts to get you started on the topic:

• What was the state of Soviet invasion plans for Japan during the end of WWII?
  ^(10 Aug 2015 | 3 comments)
  ^(/u/The_Alaskan describes the topic in context with the known lack of Japanese defenses on Hokkaido.)

• How credible was the threat of a Red Army invasion of the Japanese Home Islands at the end of WWII?
  ^(09 Jan 2014 | 32 comments)
  ^(/u/Acritas cites Russian primary sources on the topic which are later translated.)