Allied leaders met in three (well, four considering Potsdam) conferences to discuss the future after the war -- the Casablanca conference in 1943, the Tehran Conference in 1943, and Yalta Conference in 1945. However, looking at the map, these locations seem problematic.
Tehran, being in Iran, seems (relatively?) easily accessible for Stalin, but how did Churchill and Roosevelt cross Europe (mostly controlled by axis), and parts of Asia to reach Tehran? Same question with Yalta, especially since Eastern Europe was being teared apart in 1945 by the approaching Red Army, and the retreating Nazi Army.
How were the locations decided, and how did each of 3 nations ensure that the respective leaders could reach safely to and from the conference venues?