Well Actually 3 things happened which caused Japan to surrender (the most direct things) 1 was they atomic bomb and the Japanese knew about the E equals MC square equation. The Soviet invasion came later and then there was the coup of the military that failed.
The problem with history is that we cannot really replay by taking away 1 of the 3 and go back to 1945 and see what happens. Which makes it a lot harder than e.g. make a different nuke and see if it explodes. Historians canât really redo the steps.
We know that Japan at the time was deeply religious and was hoping for a repeat for of the 13th century invasion attempts (which failed). Also they had a deep fear of being colonized like the rest of the Far East AND pursued a in my view understandable strategy of making any invasion prohibitively expensive. The US forces suffered massively as well in Okinawa, Saipan and Iwo Jima and and invasion would be very difficult. The United States/UK were the ones with excellent naval power.
So the Soviets could invade Manchuria but I wonder whether they could have successfully invaded Japan. Some historians point out that they took Sakhalin but that was AFTER the surrender. Had the Soviets or US/UK +Allies suffered a failed attempt one can wonder whether a peace without invasion would be an option. Note everyone in the US and UK was interested in a war which could last until 1948 leading to many thousands additional coffins while massive amounts of resources went to rebuilding Europe and âthe coloniesâ. The latter was a big priority and the decolonization was already starting.
The arrival of the atomic bomb was a science fiction weapon of enormous power. It was slated to use as a tactical weapon which could lead to massive instant deaths and radiation poisoning. Suddenly the US had a wonderweapon and it was unlikely that they would have been demoralized.
Here is an excerpt of the speech of Hiro-Hito, sadly many writers sum up the horrors of the bombs yet just plainly âforgetâ to write about the speech. The way the US forces had been slaughtering Japanese military and also civilians in 1945 makes the speech below understandable.
Excerpt on the bomb, link to full speech below
"Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, it would not only result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.
Such being the case, how are we to save the millions of our subjects, nor to atone ourselves before the hallowed spirits of our imperial ancestors? This is the reason why we have ordered the acceptance of the provisions of the joint declaration of the powers.
We cannot but express the deepest sense of regret to our allied nations of East Asia, who have consistently cooperated with the Empire toward the emancipation of East Asia."
Speech by Emperor Hirohito accepting the Terms of Surrender, 14 August 1945 (mtholyoke.edu)
Finally just to be clear, the Soviets did help massively to defeat Nazi Germany as well a many smaller countries. Also to be fair the Japanese donât give any context on why the bombs were dropped and portray themselves as the liberators of Asia which in a sense is true because they did trigger the decolonization and are right to ask the question âWhat were the Dutch/British/French doing in my neck of the woodsâ. I know Dutch history likes to concentrate on the Japanese crimes but is a lot less forthcoming about the horrors in Indonesia, Suriname, India and etc etc. Had I been Japanese I would have supported kicking the Westerners out of Asia, their museums were a big eye-opener for me.