The Soviet Union occupied Central and Eastern Europe, and the Red Army effectively controlled the Baltic States, Poland, Czechoslovakia, Hungary, Bulgaria and Romania. Refugees have fled these countries. Stalin had installed a communist puppet government in Poland, insisted that his control of Eastern Europe was a defensive measure against possible future attacks, and claimed that this was a legitimate sphere of Soviet influence.  Little real progress was made in Potsdam beyond an agreement to implement the commitments made in Woalta. Poland`s border became the Oder and Neisse to the west, and the country received part of the former East Prussia. This necessitated the transfer of millions of Germans in these regions to Germany. The governments of Romania, Hungary, and Bulgaria were already controlled by the Communists, and Stalin stubbornly refused to let the Allies interfere in Eastern Europe. In Potsdam, Truman told Stalin about the “new weapon” of the United States (the atomic bomb) that it wanted to use against Japan. On July 26, Japan received an ultimatum calling for unconditional surrender and threatening harsher airstrikes. After Japan rejected this ultimatum, the United States dropped atomic bombs on Hiroshima and Nagasaki.
The Germans of Czechoslovakia (34% of the population of the territory of the current Czech Republic), known as Sudeten Germans, but also Carpathian Germans, were expelled from the Sudetenland, where they formed a majority, from the linguistic enclaves of Central Bohemia and Moravia, and from the city of Prague. One of the most controversial topics of the Potsdam Conference concerned the revision of the German-Soviet-Polish borders and the expulsion of several million Germans from the disputed territories. .