Japan hoped the Soviet Union would mediate peace talks with the Western Allies in 1945.
Before the Soviet declaration of war, Japanese leaders viewed Moscow as a possible intermediary because the Soviet Union was still officially bound to Japan by the Soviet–Japanese Neutrality Pact. Japan hoped Soviet diplomats could help negotiate conditional surrender terms with the United States and the United Kingdom.
The strategy failed because the Soviet Union had already promised the Western Allies that it would enter the war against Japan. On 8 August 1945, Moscow declared war, and Soviet forces invaded Japanese-held Manchuria shortly afterward on 9 August. The Soviet offensive destroyed Japan’s expectation that the Soviet government would act as a neutral mediator.
Japan’s diplomatic calculations changed alongside the atomic bombings of Hiroshima and Nagasaki. The Soviet entry into the war became one of the major pressures in the final decision to accept the Potsdam Declaration. Japan announced that acceptance on 15 August and signed the formal surrender on 2 September.