I voted for the Soviet invasion of Manchuria.
Prior to the dropping of the first atomic bomb, the Japanese were already willing to agree to a conditional surrender.
The Soviet Union was fundamentally hostile to the Western democracies. Some Japanese leaders hoped for an alliance between Japan and the Soviet Union–an alliance against the U.S. and Britain. Their hope was that the Soviet Union would enter the war on Japan’s side.
The Soviet invasion of Manchuria demonstrated that any kind of diplomatic arrangement with the Soviet Union was no longer an option. With its diplomatic options exhausted, Japan’s only way of avoiding the Americans’ demands was to achieve some kind of military victory. There was a shortage of Japanese military victories from '43 - '45, and the Soviet invasion of Manchuria only made things worse. That invasion was regarded by both sides as a prelude to the invasion of Japan itself. The quick collapse of Japan’s Manchurian forces inspired little hope that Japan would do well in a follow-up Soviet invasion of its home islands. Still less did it seem possible for Japan to defeat a combined invasion of American and Soviet forces. At that point, it was concluded that it was better to unconditionally surrender to the Americans, than to have unconditional surrender imposed anyway by a combined Soviet-American force.
Why wasn’t Stalin interested in joining Japan’s war against the U.S. and Britain? I see two main reasons: 1) He wasn’t yet ready for such a war, and 2) By 1945, Japan’s military had little left to offer. By the early '50s, Stalin may have felt himself ready or close to ready for war against NATO. Some historians feel that Stalin permitted the Korean War to proceed because he wanted to test American military readiness. The results in that war were such that Stalin felt comfortable moving forward with his plans for WWIII. However, he died before putting those plans into effect.