Churchill, Roosevelt, and Stalin at the Yalta conference, February 1945 (Molotov is visible on the left) [Public domain]
The conference took place at Yalta in the Crimea between 4 and 11 February 1945. The purpose was to lay out the groundwork for postwar Europe—Poland in particular.
Stalin agreed to free elections in Poland (a promise that he later broke) and, in their turn, the Russians obtained agreement from Britain and the United States to use their air superiority to bomb lines of communication in the east of Germany, notably in the Berlin-Dresden-Leipzig area. (This would doom Dresden which, at that point, had suffered only minor damage from bombardment from the air.)