Atomic bomb drop on Hiroshima Aug 6th 1945. Before this, the decision making group had not even met. “The highest decision-making body was not even convened after Hiroshima. The cabinet was divided. The atomic bomb was effective enough that for the first time, cabinet decision-makers decided to really terminate the war. But on what conditions, they were totally divided.” - Tsuyoshi Hasegawa, 2005, “Racing the Enemy: Stalin, Truman and the Surrender of Japan.”

Soviet Union declares War on Japan Aug 8th, 1945

Soviet Union invades Manchuria and the US drops a second Atomic device on Nagasaki Aug 9th, 1945.

So the Soviet Union was still talking peace with Japan when the first bomb was dropped and the government had not even met about surrender prior to the first device. To claim it was the the Soviets alone is not supported by the data.

In the end the Japanese refused to even discuss surrender prior to the first bomb. They considered it but refused to meet to discuss it till after. Then their one lifeline declared war instead of helping them out and that same day the second device was used.

You could argue that the second device was not needed in hindsight... But there was no cooperation nor communication between the US and the Soviets on these issues.

The most likely scenario is the Japanese knew they had lost clearly by early Summer 1945. They were willing to go down fighting till the Allies showed they had a totally new weapon and this encouraged them to consider surrender, the double punch of us showing it was not an isolated incident (and lying saying they had dozens of these weapons) and the Soviets invading made them not just come to the table, but surrender.

There is a significant difference between defeat and surrender. The Japanese were defeated, but still wanted to fight.