Ideological Differences
The Soviet Union and her wartime allies, chiefly the United States and Britain, had basic differences in their political values and in their opinions about the nature of society. The US had adopted capitalism, while the USSR adopted communism. It was unlikely that these two opposing political systems could cooperate genuinely after their wartime enemies, the Axis powers, were wiped out.
Both Sides claimed that their ideas and ideologies were right. Each criticized the other for trying to extend its system all over the world.