Under terms of Yalta agreement, the Soviets declared war on Japan on the night of August 8, 1945, exactly three months after the defeat of Germany in Europe. Up until that time the Soviets had a neutrality pact with Japan.
The Soviets immediately invaded Japanese-held Manchuria and northern Korea and ended up occupying these areas after the war. The Soviet invasion started just before the dropping of the second atomic bomb and may have been a deciding factor in the Japanese surrender. Up until that time, Japan had hoped the Soviets would act to arrange a negotiated settlement to the war with the US, and the Soviets did not discourage that idea until the time of the actual invasion.
The US government was very anxious to get the Soviets into the war to reduce the likely casualties from an invasion of Japan, and the agreement to get the Soviets into the war with Japan came several months before the successful US test of the atomic bomb.
A result of the Soviet invasion was they ended up occupying northern Korea and northern China after the end of the war. The Soviets used the occupation to set up a Communist government in North Korea, and the Soviet areas of occupation in northern China became a base of operation for the Communist Chinese.
Do you think the Japan would have surrendered without the Soviet invasion?
At the time that Roosevelt made the agreement with the Soviets, the atomic bomb was still untested. Do you think that getting the Soviets to declare war would have prevented the need for a US invasion even if the atomic test had been a dud?
Did Roosevelt effectively agree to the creation of North Korea by getting the Soviets to declare war on Japan?
Did you think that the Soviet occupation of Manchuria was a deciding factor in the Communist victory over the Nationalist government in 1949?
For background see: