The Iron Curtain was the political and physical boundary that divided Europe from the end of World War II in 1945 until the end of the Cold War in 1991. East of the Iron Curtain were the smaller states controlled by the Soviet Union, in 1955 formally allied by the Warsaw Pact. Many nations to the west of this geopolitical divide were NATO members. Over time these economic and military alliances developed into broader, more entrenched, cultural barriers that deepened widespread distrust on both sides. Initially, the term "Iron Curtain" was a literal description of physical barriers such as razor wire, fences, walls, minefields, and watchtowers along the borders of the opposing powers. But the term eventually took on a broader, symbolic meaning perceived as a generalized "differentness" of ideology, economy, government, and way of life that emerged when the Cold War severed earlier cultural connections between European populations.

The term's origin is often attributed to the speech "Sinews of Peace" delivered by Winston Churchill on 5 March 1946 in Fulton, Missouri where he said: "an iron curtain has descended across the Continent. Behind that line lie all the capitals of the ancient states of Central and Eastern Europe." In fact, the phrase was originally used by Queen Elisabeth of Belgium, when in 1914 when she described an "Iron Curtain" descending between her people and the nation of Germany.

The nations to the east of the Iron Curtain were Poland, East Germany, Czechoslovakia, Hungary, Romania, Bulgaria, Albania, and the USSR; however, East Germany, Czechoslovakia, and the USSR have since ceased to exist. Countries of the USSR were the Russian SFSR, Byelorussian SSR, Latvian SSR, Ukrainian SSR, Estonian SSR, Moldavian SSR, Armenian SSR, Azerbaijan SSR, Georgian SSR, Uzbek SSR, Kirghiz SSR, Tajik SSR, Lithuanian SSR, Turkmen SSR, and Kazakh SSR. Events that demolished the Iron Curtain started with the Fall of communism in Poland, Hungary, East Germany, Bulgaria, Czechoslovakia and Romania.

Due to the decreased human activity around the physical border during the Cold War, natural biotopes were formed, now the European Green Belt. With the exception of the Kars-Gyumri railway crossing which operated during the Soviet Era, the Turkish–Armenian border has remained closed since the 1920s and is sometimes described as the Iron Curtain's last vestige.

In the 1948, iron safety curtains were installed on theater stages to slow the spread of fire.

Perhaps the first recorded application of the term "iron curtain" to Soviet Russia was in Vasily Rozanov's 1918 polemic The Apocalypse of Our Time. It is possible that Churchill read it there following the publication of the book's English translation in 1920. The passage runs:

With clanging, creaking, and squeaking, an iron curtain is lowering over Russian History. "The performance is over." The audience got up. "Time to put on your fur coats and go home." We looked around, but the fur coats and homes were missing.

In 1920, Ethel Snowden, in her book Through Bolshevik Russia, used the term in reference to the Soviet border.