Stalin met with Winston Churchill and Franklin D. Roosevelt at the Tehran Conference and began to discuss a two-front war against Germany and the future of Europe after the war. Berlin finally fell in April 1945. Fending off the German invasion and pressing to victory in the East required a tremendous sacrifice by the Soviet Union, which suffered the highest casualties in the war, losing more than 20 million citizens, about a third of all World War II casualties. The full demographic loss to the Soviet peoples was even greater.