"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."
And on top of this, the Japanese had been negotiating their surrender with the ostensibly neutral USSR hoping to get more favorable conditions with their assistance. And the US had been secretly intercepting these cables and knew all about it. [2] And on top of this the US had also already created another secret agreement with the USSR such that they would declare war on Japan on August 8th, which would end any hope at all for Japan. We nuked them on August 6th, and then again on the 9th.
It was completely unnecessary.
[1] - https://nsarchive.gwu.edu/document/28466-document-47-trumans...
[2] - https://nsarchive.gwu.edu/document/28458-document-39b-magic-...