All these leaders:
- Adm. William Leahy (President Truman’s Chief of Staff)
- Henry Arnold (US Army Air Forces commanding general)
- Adm. Chester Nimitz (Commander in Chief of the Pacific Fleet)
- Maj. Gen. Curtis LeMay, (head Bomber Command)
- Gen. Dwight Eisenhower
are all on record as acknowledging that "Japan was already defeated". 
 Japan knew they could not win and were concentrating on obtaining the best possible terms of surrender, hoping that Stalin might be convinced to mediate a settlement between the United States and Japan.
This hope was dashed by the Soviet declaration of war and invasion of Manchuria and Sakhalin Island, which happened at 1 minute past midnight on August 9, 1945 .
Another important factor is that Japan had good reason to be wary of USSR military might .
It's arguable that Japan's fear of fighting the USSR is the reason Japan went east (to Pearl Harbour) instead of west (to eurasia) in Dec. 1941 .
 and  are thoroughly researched and make compelling arguments.