The Soviet Union declared war on Japan on 8 August 1945, before the formal surrender.
The declaration followed a Soviet promise made at the Yalta Conference to enter the war against Japan after Germany’s defeat. Soviet forces attacked Japanese positions in Manchuria shortly after midnight on 9 August, opening a major offensive against Japan’s Kwantung Army.
This development came between the atomic bombings of Hiroshima and Nagasaki. Japanese leaders faced pressure from several directions: Allied bombing, the atomic attacks, the Soviet entry into the war, and the prospect of invasion.
The date can cause confusion because the Soviet offensive began after midnight in East Asia, on 9 August local time, while the declaration itself was delivered in Moscow on 8 August. Japan announced its decision to surrender on 15 August.