At the Tehran Conference in Nov 1943 and at the Yalta Conference in Feb 1945, Joseph Stalin had agreed declare war on Japan three months after Germany would be defeated. On 5 Apr 1945, the Soviet Union informed Japan that the Soviet Union would not renew the Soviet-Japanese Neutrality Pact of 1941, which ensured non-aggression between the two nations through 13 Apr 1946. At 2300 hours Transbaikal time on 8 Aug 1945, Soviet foreign minister Vyacheslav Molotov informed Japanese ambassador Sato that the Soviet Union was revoking the neutrality pact with a declaration of war effective on 9 Aug; at this point, the neutrality pact was still six months from its natural expiration.
At one minute past midnight on 9 Aug 1945, or 61 minutes after the declaration of war, Soviet troops in three fronts poured into Japanese-occupied northeastern China, a region known as Manchuria that had been governed by the puppet regime of Manchukuo since 1932. The Soviet troops were of the Far Eastern Command under the overall command of Marshal Aleksandr Vasilevsky, who envisioned a giant pincer movement. The Transbaikal Front under Marshal R. Y. Malinovsky was to attack from the west across the Inner Mongolian desert and over the Greater Khingan mountain range, with Mukden as the primary target; the attached 36th Army was to break off after the initial invasion and head toward Harbin and Qiqihar to meet the 2nd Far East Front. The 2nd Far East Front under General M. A. Purkayev attacked in the center largely in a support role only, with the primary objectives of securing Harbin and Qiqihar, upon the successful completion of which, the front was to move toward Port Arthur after the 1st Far East Front completed its primary objectives. Finally, from the east over the Lesser Khingan mountain range, the 1st Far East Front under Marshal K. A. Meretskov was to capture the cities in eastern Manchuria, including Changchun; its secondary objective was to cut off Japanese escape routes into Korea, and its tertiary objective was to invade and occupy northern Korea. In total, 1,577,725 men in 89 divisions with the support of 3,704 tanks, 1,852 self-propelled guns, 27,086 artillery pieces, and 3,721 aircraft were utilized in the invasion.