History of Europe
Communism became less attractive when it became clear that it could promote economic growth less effectively than the capitalistic Western states and that it was not suited for a reform that allowed freedom of speech for everyone.  Therefore, the Soviet Union forced Hungary to give up its reform in 1956, it favoured the building of the Berlin Wall in 1961 and it stopped reform in Czechoslovakia in 1968.  When in 1988/89 the Soviet leader Gorbachev made clear that he would not force the countries of the East block to stick to Communism the Berlin Wall was torn down in 1989 and the Soviet Union collapsed (1991). Then the United States was the only superpower left.