As with any global conflict, there was no single cause of the Cold War - there were multiple causes that centred around Communism. The USSR and China, two major world powers, were both Communist, while the US was suspicious of Communists.
These factors helped contribute to a climate of suspicion and hostility between East and West, which grew stronger towards the end of the 1940s at the time of the Soviet blockade of Berlin and the Berlin airlift.
See also: Cold War Chronology
"Causes of the Cold War in 1945". HistoryLearning.com. 2023. Web.