One Soviet precondition for a declaration of war against Japan was an American official recognition of Mongolian independence from China (the Mongolian People's Republic had been a Soviet satellite state from its beginning in 1924, through World War II). The Soviets also wanted recognition of Soviet interests in the Manchurian railways and Port Arthur (but not asking the Chinese to lease). These conditions were agreed to without Chinese participation.

The Soviets also wanted cession of Karafuto and the Kuril Islands, taken from Russia by Japan in the Russo-Japanese War (1905), which was also agreed to.

In return, Stalin pledged that the Soviet Union would enter the Pacific War three months after the defeat of Germany. Later, at Potsdam, Stalin promised President Truman to respect the national unity of Korea which would be partly occupied by Soviet troops.

Furthermore, the Soviets agreed to join the United Nations, given the secret understanding of a voting formula with a veto power for permanent members of the Security Council, thus ensuring that each country could block unwanted decisions.
https://en.wikipedia.org/wiki/Yalta_Conference