Why did countries have to choose an alliance to one of the superpowers after World War 2?

After World War Two, tensions were high between the western world and Communism society. Events spiraled out of control and eventually there was a threat of nuclear warfare. To protect themselves against this 2 sides were formed The Warsaw Pact was the Communist group and NATO was the western group. Both groups offered the same ideas in terms of protection against invasions and long range nuclear missiles. Some countries, though, did not sign with either group such as Switzerland