I have heard this before about the Soviets taking the northern half of Japan (a al Korea) if the war continued, but what I want to know is HOW they would get there if the United States Navy simply said - NO.
There has never been a Navy of the Ages as powerful as the USN in 1945. The with the WWII in Europe over, the British assembled one of the most powerful units they could assemble, Naval Task-Force 57 to send to assist the US Navy in the Pacific against the Empire of Japan. This British TF-57 was lost in a sea of American ships.
The USSR had no Navy to speak of in the Pacific...
Unless the Soviets could walk on water, they were not getting to Japan without US permission...
dvwjr
Our navy was not about to attack any Russian ships given the tenuous hold that we had in Germany. Russia simply occupied what they want while we stood by in the Balkans, Poland, Germany, Japan and elsewhere. The shock and awe of the A-bomb was essential in stopping the Russian advance everywhere and is likely what kept Russia out of Austria also.