structural:summarization
Building antagonism

The antagonism between the Soviet Union and the West that came to be described as the "iron curtain" had various origins.

During the summer of 1939, after conducting negotiations both with a British-French group and with Nazi Germany regarding potential military and political agreements, the Soviet Union and Nazi Germany signed the German–Soviet Commercial Agreement (which provided for the trade of certain German military and civilian equipment in exchange for Soviet raw materials) and the Molotov–Ribbentrop Pact (signed in late August 1939), named after the foreign secretaries of the two countries (Vyacheslav Molotov and Joachim von Ribbentrop), which included a secret agreement to split Poland and Eastern Europe between the two states.

The Soviets thereafter occupied Eastern Poland (September 1939), Latvia (June 1940), Lithuania (1940), northern Romania (Bessarabia and Northern Bukovina, late June 1940), Estonia (1940) and eastern Finland (March 1940). From August 1939, relations between the West and the Soviets deteriorated further when the Soviet Union and Nazi Germany engaged in an extensive economic relationship by which the Soviet Union sent Germany vital oil, rubber, manganese and other materials in exchange for German weapons, manufacturing machinery and technology. Nazi–Soviet trade ended in June 1941 when Germany broke the Pact and invaded the Soviet Union in Operation Barbarossa.

In the course of World War II, Stalin determined to acquire a buffer area against Germany, with pro-Soviet states on its border in an Eastern bloc. Stalin's aims led to strained relations at the Yalta Conference (February 1945) and the subsequent Potsdam Conference (July–August 1945). People in the West expressed opposition to Soviet domination over the buffer states, and the fear grew that the Soviets were building an empire that might be a threat to them and their interests.

Nonetheless, at the Potsdam Conference, the Allies assigned parts of Poland, Finland, Romania, Germany, and the Balkans to Soviet control or influence. In return, Stalin promised the Western Allies that he would allow those territories the right to National Self-Determination. Despite Soviet cooperation during the war, these concessions left many in the West uneasy. In particular, Churchill feared that the United States might return to its pre-war Isolationism, leaving the exhausted European states unable to resist Soviet demands. (President Franklin D. Roosevelt had announced at Yalta that after the defeat of Germany, U.S. forces would withdraw from Europe within two years.)