The phrase is frequently used in relation to the cold war

Iron Curtain

Explanation:

Former British Prime Minister Winston Churchill famously called the divide between the democratic western and communist eastern parts of the world the "Iron Curtain" in one of his very notable speeches.

The Cold War was a period of tense conflict, without any outright arms fighting, between democracy and communism (although the two main contenders were the US and the Soviet Union). The Iron Curtain was a symbol of this divide.

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