The first reason is that Roosevelt’s death in April 1945 brought an end to any superficial unity that still existed at the end of World War 2 in 1943. Truman was now the American President, and relations between him and Stalin were deteriorating very quickly indeed, especially when Stalin was such a determined character and Truman was less willing to compromise, compared to Roosevelt.
The second reason is that America had developed the atomic bomb in June 1945. This played a major part in the change in Truman’s attitude at the Potsdam Conference in the same year. The successful development of the atomic bomb had lifted Truman on his high horse, and this links back to his attitude that clashed with Stalin. Now Truman was even more arrogant, and as a result, it made Stalin even more determined to get his way.
The third reason is that as a result of Churchill’s Iron Curtain Speech, the Truman Doctrine and Marshall Aid, Stalin had responded to these by forcing communism on Eastern Europe, then Comecon and Cominform, and hence the Berlin Crisis. This point marks the peak of the Cold War when relations between the USSR and the USA were at their worst. This was because of the Berlin Blockade, when America had humiliated Stalin so devastatingly, with Stalin on the verge between firing at the airplanes, or ignoring them and keeping the blockade up. This is another example of Stalin’s stubborn nature, as seen in the Potsdam Conference mentioned in the previous paragraph.
Finally, the fourth and last reason is that as a result of the Berlin Crisis, President Truman had signed the NATO agreement in 1949, which was a clear union of America and Western Europe against the USSR and Eastern Europe, and most importantly, against Communism. This made Stalin furious, and he responded in defence with the Warsaw Pact in 1955. This move shows that he is not to be outdone, and hence here at the peak of tension and conflict from 1943-56 we see the complete change that has occurred in the relation between the two powers.