How did the alliances that formed in Europe after world war 2 form an "iron curtain" as described by Winston Churchill in a 1946 speech?

It wasn't the Allies that created the "Iron Curtain." The Iron Curtain was created by the Soviet Union, then led by Joseph Stalin. He took all the Eastern European countries of Slavic decent and forced them to be a communist state. The true fault really lies with FDR who did not listen to Churchill when they had attended the "Big Three" meeting at Yalta. The Allies agreed to give Russia dominion over the Eastern Slavic countries (countries such as Poland, Ukraine, Belorussia, and others) without really knowing what they had been just sold into.