Meanwhile having denounced their neutrality agreement in early April the Soviets declared war on Japan on August 8th, and were poised to attack their northernmost home island of Hokkaido in a matter of days. The Japanese simply did not have time to counter this threat, nor could they defend themselves simultaneously on two fronts.
Dropping atomic bombs on Hiroshima and Nagasaki may have provided a dramatic exclamation point to ending the war with Japan, but from a strictly military perspective wasn't necessary. The Japanese ran out of strategic options when the Soviet entered the war against them.
If it wasn’t for the bomb Japan would have been divided like Korea, and the resulting Civil War would have made the Korean Conflict look like Child’s Play.