I often see people on reddit claiming that the bombing of Hiroshima and Nagasaki did not end the war in the Pacific, but rather, the Japanese surrendered because they were afraid of an invasion by the Soviet Union.
So, is there any truth to this? From my understanding, the Soviet Union had a very small naval presence in the Pacific theater. According to wikipedia: "By August 1945, the Pacific Fleet consisted of two cruisers, one destroyer leader, ten destroyers, two torpedo boats, 19 patrol boats, 78 submarines, ten minelayers, 52 minesweepers, 49 "MO" anti-submarine boats (MO stands for Малый Охотник, or "little hunter"), 204 motor torpedo boats and 1459 war planes." I just don't see how it could be possible for the Soviet Union to launch an amphibious invasion of Japan with that force.
I'm moving right now, so I don't have my books handy, but I recall a reference in Richard Frank's Downfall: The End of the Imperial Japanese Empire.
In short, the Soviet Union knew Japan was close to surrender. The commander on the spot told Stalin in early August that he had plans for an invasion drawn up and would be ready to invade by the end of the month. In short, the invasion would've been a slapdash affair intended to throw a motor rifle division ashore, followed by two others on Hokkaido.
With no specialty landing craft available, the plan involved physically ramming freighters ashore and unloading troops over the side. Follow-on forces would have come through ports seized after the initial landing. Trawlers, destroyers and minesweepers would have carried soldiers ashore with small boats.
Japanese resistance was thought (correctly) to be almost nonexistent. Almost every soldier, plane and ship Japan had was in Kyushu or Honshu, awaiting the American invasion. The American strategic bombing and strategic mining campaign made inter-island movement almost impossible, which meant the handful of undersupplied soldiers on Hokkaido would have been left to fend for themselves.
Would it have been enough? I don't know. The Soviet Union proved itself capable of island-hopping in the Kurils against limited opposition, but it was not an extended campaign, and I don't know how good Soviet logistics would have been in a protracted campaign.
From what I understand of the Soviet plan, it seems to have been drawn up as a contingency: If Japan suddenly collapsed and Stalin ordered, the commander had a way to grab a foothold on Hokkaido before the end of the war. That would have given the Soviet Union a seat at the bargaining table for any postwar settlement.
It was recently discussed here. There are other good answers there too.