Why was there so much mistrust between the Soviet Union and the West by the end of the Potsdam Conference?

Why was there so much mistrust between the Soviet Union and the West by the end of the Potsdam Conference?

During the Second World War, the USA, Britain and communist Russia had been close Allies in the common goal of defeating Hitler’s Nazi Germany. At the Yalta Conference of February 1945 just before the end of the war, the so-called “Big Three” of Stalin, Churchill and Roosevelt had agreed on many issues involving post-war Germany and the division of Eastern Europe. However at the Potsdam Conference in August of that same year, much had changed within the wartime Alliance. Germany had surrendered in May 1945 and with the prospect of peace the once close alliance began to fail. One of the main reasons for this was the change in leadership for both the USA and Great Britain. Between the conferences at Yalta and Potsdam, President Roosevelt had died and been succeeded by Harry Truman. At around the same time Winston Churchill had been voted out in the 1945 general election and was replaced by Clement Attlee of the Labour Party. Both these new leaders were highly suspicious of Stalin’s intentions in Eastern Europe and therefore mistrusted him greatly. Stalin, meanwhile saw this change in leadership as a chance to extend Russia’s territory in Eastern Europe. He was painfully aware of the damage caused by the German invasion in 1941, and wanted to prevent anything of the sort ever happening again. His plan was to occupy a number of “satellite” states in Eastern Europe and therefore create a ‘buffer’ which could bear the brunt of any attack. He thought he could achieve this by bullying Truman and Attlee who were both new to their jobs and therefore probably insecure and, possibly, weak. This however was not the case, as during the Potsdam Conference, President Truman was informed of a successful test of the new Atom Bomb – something Stalin knew nothing about. Now Truman had this new ‘secret weapon’, he became much more forceful towards Stalin and started ordering proceedings. When Stalin objected, Truman announced that a successful test of an Atomic Weapon had been completed. This filled Stalin with fear and mistrust, as he could now no longer rely on Russia’s natural geography as a defense. After the Atom Bomb had been dropped on Japan and ended the war in the Pacific, Stalin poured massive amounts of money into his own Atomic program at a time when Russia was in desperate need of rebuilding, both physically and economically.