read this.

At Yalta, Roosevelt and Churchill discussed with Stalin the conditions under which the Soviet Union would enter the war against Japan and all three agreed that, in exchange for potentially crucial Soviet participation in the Pacific theater, the Soviets would be granted a sphere of influence in Manchuria following Japan’s surrender. This included the southern portion of Sakhalin, a lease at Port Arthur (now Lüshunkou), a share in the operation of the Manchurian railroads, and the Kurile Islands.

https://history.state.gov/milestones/1937-1945/yalta-conf