Which event is characterized as the start of heightened geopolitical tensions as expressed in Churchill's speech?

The answer indicating the onset of the Cold War is correct because Winston Churchill's famous "Iron Curtain" speech in 1946 clearly identified the growing divide between the Western democracies and Eastern communist countries led by the Soviet Union. This speech epitomized the fears of a bipolar world emerging from the aftermath of World War II, where ideological and political tensions escalated significantly. Churchill argued that an "Iron Curtain" had descended across Europe, symbolizing the boundary between the free nations of the West and the communist-controlled nations of the East. This marked a definitive moment in which the geopolitical landscape shifted, leading to an extended period of rivalry and conflict known as the Cold War, characterized by military buildup, ideological competition, and proxy wars.

The other events mentioned, while significant in their own contexts, do not align with the moment captured in Churchill's speech. The beginning of World War I happened decades earlier and does not relate to the post-World War II dynamics. The Berlin Wall, established in 1961, was a symbol of the division resulting from the Cold War rather than its beginning. The formation of the Warsaw Pact in 1955 was a response to NATO and further entrenched the division created by the Cold War but occurred after the tensions had