The Cold War was rooted in the collapse of the American-British-Soviet alliance that defeated the Germans and Japanese during the Second World War. Already divided ideologically and deeply suspicious of the other side’s world plans, American and British diplomatic relations with Joseph Stalin’s Soviet Union severely cooled after the war, over several items. In particular, the Soviets placed and kept local communist parties in power as puppet governments in once-independent countries across Eastern Europe, without due democratic process. This situation led former British Prime Minister Sir Winston Churchill to state on 5 March 1946 that an “iron curtain” had descended across the European continent.

Reference: https://www.thecanadianencyclopedia.ca/en/article/cold-war