Answer :
Declaring war on Japan that once the Nazis were defeated was what the Stalin agreed to do in return for the Allied invasion of France.
declare war on Japan once the Nazis were defeated
the USSR agreed to aid in the Pacific War with a land invasion and defeat of the Nazis.
With a land invasion and defeat of the Nazis, the USSR could turn their troops on the Eastern Front to invading China and pushing the Japanese back out of China and Manchuria. The USSR agreed and followed through but were not needed because the US used the atomic bombs and received unconditional surrender from Japan.