Before the end of the World War Two, the relationship between the US, Britain and USSR went on very well. However, the attitudes of USSR and US had a big U-turn from the Potsdam conference since US saw the communists as a big threat to them. Eventually by early 1946, The Long Telegram laid the basis for the policy of containment against communism.
One specific reason why there was such a big change from the Potsdam conference is that the leaders of Britain and the US were different. Harry Truman who had been working as the vice president before the death of Franklin Roosevelt was now the US president. However, Truman wasn't prepared for this position, this gave Truman a sense of weakness, which was increased by the growing pressure exerted within the USA by anti-communist groups. The result of Truman trying to assert his authority was the adoption of a more openly hostile attitude towards the Soviet Union.
He was also very suspicious about Stalin's aim and political ideas.
Firstly, it was because they both had very different political systems. The US was a very democratic and capitalist country, but on the other side, the USSR had the communist system. The idea of communism meant economical corruption to Truman and American people. George Kennan, Deputy Chief of Mission in the US Embassy in Moscow, sent the Long Telegram to Truman in February 1946 and it analyzed the Soviet foreign policy which emphasis on the role of communist ideology. Kennan saw the leadership of the USSR as suspicious and aggressive, threatening the USA. When Truman read this, he's attitude towards Soviet Union was then even stronger. He wanted to safeguard US's interest, so the basis of Containment was set up. He also didn't want the mistake that Chamberlain did in...