What was the iron curtain? why were western leaders so concerned about it?

The iron curtain refers to the line that separated the Soviet satellite states of eastern Europe from western Europe. The term "iron curtain" was chosen to describe the degree of separation that existed between Communist and non-Communist states in Europe and to describe the repression and force by the USSR in the establishment of its sphere of influence. I think that western leaders were so concerned about the Iron Curtain because it represented the Soviet Union's continued desire to expand, the threat of war with the Soviet Union, and the overall threat of Communism.