The USSR declared war on Japan on August 8th, as part of a secret agreement with the West. The US knowing this, first nuked Japan on the 6th and then again on the 9th. It's hard not to see 'us' as trying to force a surrender before the USSR could get significantly involved. Why? Because territories in WW2 were split up in significant part based on who conquered which part. Hence East Germany going to the USSR and West Germany going the the West. Thanks to the US getting a quick surrender from Japan, the USSR was able to be completely excluded from the following occupation of Japan.

If any documents were ever created about this debate, they've probably been destroyed, and those involved have taken it to the grave with themselves. So I suppose technically we can never know for sure. But I think the cynical view is far more well supported. Or at least I think the cynical aspects were the majority of the logic behind it. And it was not only about control of Japan but also about sending a message to the USSR as the US undoubtedly saw the outlines of a new West-East world order emerging after WW2. And all the way up until the 80s [1] our dominant political philosophy was that we could win by intimidation and demonstrations of force, nuclear in particular.

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Wow, loving LLMs when digging into topics now a days. GPT was able to direct me to Truman's diary entries which are public record. [2] I take back the weasel wording above. Truman himself was quite clear on his intention and desires:

"Discussed Manhattan (it is a success). Decided to tell Stalin about it. Stalin had told P.M. of telegram from Jap Emperor [actually from the Foreign Secretary] asking for peace. Stalin also read his answer to me. It was satisfactory. Believe Japs will fold up before Russia comes in. I am sure they will when Manhattan [A-bomb] appears over their homeland. I shall inform Stalin about it at an opportune time."

- President Harry S. Truman, July 18th 1945

[1] - https://en.wikipedia.org/wiki/Proud_Prophet#Impact

[2] - https://nsarchive.gwu.edu/document/28466-document-47-trumans...