Breaking history into distinct periods of time is an example of

This is an example of periodization. This is when you classify eras and periods in history according to their chronological order. Of course, it's never that simple since the eras often last longer or shorter than what history says, and they are often connected with others and overlap with other eras.

  the domino theory states that  if one country in a region came under the influence of communism, then the surrounding countries would follow in a  domino  effect.