At Yalta FDR gave lots of Eastern Europe to Stalin in exchange for the Soviet Union joining in the war against Japan with whom they had a neutrality pact. They promised to go to war against Japan three months after the end of the war against Nazi Germany. The Germans surrendered on May 8, 1945. August 8 was exactly three months to the day after the end of the war in Europe.
Using the atomic bomb when we did prevented the Soviet Union from dividing Japan the way they divided Germany and Europe.
To note on Manchuria, FDR gave away China's right to full restitution/indemity for Manchuria. This FDR did without informing or consulting the Chinese government, and was in direct contradiction to the agreements in the Cairo Declaration.
I was not happenstance that the Communist Chinese used Manchuria as their strong point to defeat the Nationalists and eventually control the Chinese mainland.
As I see it, there was no reason to let Russia get involved with China or Japan at all. Russia had not participated in the war against Japan and Russian participation was not needed and, in fact, the atom bomb was not needed. Japan was an island nation which had been deprived of all access to the seas, and had to surrender on whatever terms. We only had to wait another couple of months.