I do think the Soviet attack was agreed upon in the Yalta conference and I donīt think Stalin would make a pact with the Japanese until he got what he wanted. Also in the Cairo conference 1943 it was decided that :
"Japan be stripped of all the islands in the Pacific which she has seized or occupied since the beginning of the First World War in 1914", "all the territories Japan has stolen from the Chinese, such as Manchuria, Formosa, and the Pescadores, shall be restored to the Republic of China", and that "in due course Korea shall become free and independent".
http://en.wikipedia.org/wiki/Cairo_Conference
Interesting. What happened?