Key Event Defining Early Cold War Tensions

In the immediate aftermath of World War II, the clash of ideologies between the United States and the Soviet Union contributed to the onset of the Cold War. A number of events and policies were pivotal in defining this tension. Among these, the establishment of military alliances and economic strategies showcased both nations' attempts to assert their influence globally.

Which of the following events is considered a key factor in solidifying the division between the East and West during the early stages of the Cold War?