The Iron Curtain is a reference to the geographic boundary along which the Soviet Union sealed itself off during the Cold War era.
The term has been commonly used since the 19th century, but became popular after it was used on March 5th, 1946, by British Prime Minister Winston Churchill in his speech delivered in the US. Churchill said, “From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the Continent.”
The post-World War II days were fraught with economic and political tensions. The USA and the USSR emerged as the two most important influencers. Most of the highly industrialized nations of the western world (NATO allies in particular) formed the Western Bloc, aligned with the US, and other nations which largely supported the USSR, and its communist ideologies formed the Eastern Bloc.
Over time, the USSR started to place economic, military, and diplomatic restrictions on the people of the Eastern Bloc, prohibiting them from interacting freely with the Western Bloc. The Iron Curtain refers to the boundary demarcating the two blocs, running roughly from Arctic Russia right across Eastern Europe to the Black Sea.
In the early years of the Cold War, thousands of East Germans fled to the West, thus breaching the Iron Curtain. In 1953, following the death of Joseph Stalin, it seemed that the Iron Curtain would be dissolved altogether, but the Communist East German authorities reacted by building the Berlin Wall in 1961. The Iron Curtain, a metaphorical division became more concrete with this physical wall and the establishment of various military installations demarcating the boundary between the two major blocs.
The tensions and strife caused by the building of the Berlin Wall and military units along this demarcation would continue for many more years and ended only in 1991.
Winston Churchills Iron Curtain speech condemning the policies of the former Soviet Union in Europe is often considered to be the first recognition of the Cold War.