• Hiroshima was bombed on August 6th.

• The Soviets invaded just after midnight on August 9th.

• Nagasaki was bombed just after 11 am on August 9th.

The bombing of Nagasaki was pushed up from August 11 due to a forecast of poor weather starting on August 10.

To answer your question, the Soviets entered the war AFTER Hiroshima was bombed and after the planning for Nagasaki had been done. There is some debate about the need to use the atomic bomb versus the need to show the Soviets that we had the bomb, but the Soviets were still fighting in Manchuria when the bombs were dropped. They only "streaked" across Korea because the Japanese had surrendered by that point and the US forces were a thousand miles away on Okinawa or Guam.