Why the countries that were allies during WW2 became enemies?

After the World War Two it was clear that fascism as ideology had been buried, it was no longer a threat. However, another ideologically based country not only remained and stood strong but after war also proved its power - Soviet Union. It did not take long for other winning side, the capitalist countries, supporting democracy to feel the threat especially since after the war so many people were unhappy with the economical state of their countries and other losses. Communism was becoming more and more popular, encouraging division between east and west.Since communism started its expansion around the Europe, socialist revolutions happened in many countries. It did not take long for countries like USA see it as a threat and worry that Soviet Union as a super power can become stronger. Marshall plan (American aid to western Europe) it was to rebuild Western European economies to make sure that people had no reason to have the socialist revolutions. That caused division between capitalist west and communist east, making countries as were once allies to be enemies.