In the 1960s, as dissent and protest swept through the West, nations of the Warsaw Pact were experimenting with reforms. But hopes for change were crushed by palace coups and, in the case of Czechoslovakia, outright invasion.

By the end of the 1960s, the United States and Soviet Union faced a choice: slow down their Cold War competition -- a process that would be called détente -- or continue an arms race that could end in all-out war.