I've heard the idea that atomic bombs were the deciding factor in ending the war is in large part American propaganda. The fact is the Soviets had far superior divisions (in both manpower and fire power) lined up against the Japanese in Manchuria, and shattered them within weeks.

From wikipedia(https://en.wikipedia.org/wiki/Soviet%E2%80%93Japanese_War), "Tsuyoshi Hasegawa's research has led him to conclude that the atomic bombings were not the principal reason for Japan's capitulation. He argues that Japan's leaders were impacted more by the swift and devastating Soviet victories on the mainland in the week after Joseph Stalin's August 8 declaration of war because the Japanese strategy to protect the home islands was designed to fend off an Allied invasion from the south and left virtually no spare troops to counter a Soviet threat from the north. Furthermore, the Japanese could no longer hope to achieve a negotiated peace with the Allies by using the Soviet Union as a mediator with the Soviet declaration of war. That, according to Hasegawa, amounted to a "strategic bankruptcy" for the Japanese and forced their message of surrender on August 15, 1945.[39][16] Others with similar views include the Battlefield series documentary,[2][11] among others, but all, including Hasegawa, state that the surrender was not caused by only one factor or event."