Stalin had expected to be invited to sign the ultimatum against Japan. This would justify the Soviet violation of the neutrality pact. But Truman's decision to issue the ultimatum without inviting Stalin prompted Stalin to move up the date of attack against the Japanese forces in Manchuria by 10 days to August 11. The race had begun.
But despite the devastation and horrific carnage of the Hiroshima blast, Japanese leaders continued to argue and to hope for Soviet mediation. The atomic bomb on Hiroshima shocked Stalin, however. Stalin was convinced that the bombing of Hiroshima would immediately lead to Japan's surrender. He thought he had lost the race. But upon learning that the Japanese government continued to seek Moscow's mediation to terminate the war, he again moved up the date of attack against Japan by 48 hours, to the midnight of August 9, Far Eastern time, which was 6 p.m., August 8, Moscow time.
When the Japanese ambassador met with the Soviets in Moscow at 5 p.m. on August 8, however, he got unexpected news. The Soviets declared war on Japan and within one hour invaded Manchuria. Several hours later, the United States dropped a second atomic bomb on Nagasaki. Again the Japanese authorities were unswayed. Despite the destruction visited upon a second Japanese city, it was the invasion by the Soviet Union that got Japanese attention. Finally, on August 14, with the Soviets gobbling up Manchuria, the Japanese, extremely fearful of falling under the influence of the Communists, tendered their unconditional surrender,