Short answers, pulled from the story.
The Soviet invasion of Manchuria began at one minute past midnight Trans-Baikal time on the 9th of August 1945, simultaneously on three fronts. Soviet foreign minister Vyacheslav Molotov had informed Japanese ambassador Naotake Sato of the declaration of war just hours earlier, on the evening of the 8th of August.
The Soviet Union invaded Manchuria to honor agreements made with the United Kingdom and the United States at the Tehran Conference in November 1943 and the Yalta Conference in February 1945, which required the USSR to enter the Pacific war within three months of Germany's defeat. Stalin also sought territorial gains in the Far East and had been covertly building up forces since 1943.
Soviet Far East Command under Marshal Aleksandr Vasilevsky totaled approximately one and a half million men across eighty-nine divisions, with 3,704 tanks, 1,852 self-propelled guns, 85,819 vehicles, and 3,721 aircraft. The three fronts attacked from the east, west, and north simultaneously.
The Kwantung Army had over nine hundred thousand men on paper but had been severely weakened by years of transfers to the Pacific Theater. Only six of its divisions existed before January 1945, most tanks were early 1930s models, and some units were rated less than fifteen percent combat ready. Japanese commanders themselves regarded none of the Kwantung Army's units as fully combat ready.
The Soviet entry into the war and the defeat of the Kwantung Army are considered a major factor, alongside the atomic bombings of Hiroshima and Nagasaki, in Japan's decision to surrender unconditionally by the 15th of August 1945. Japan had previously hoped the Soviet Union would negotiate a conditional surrender on its behalf, a hope the Soviets deliberately encouraged while preparing to invade.
Soviet forces launched amphibious landings in northern Korea on the 18th of August 1945, establishing control of the peninsula's northern half. In accordance with prior arrangements with the American government, Soviet forces halted at the 38th parallel. American troops landed at Incheon on the 8th of September 1945, and the division of Korea hardened into two separate states in 1948.