The Twentieth Century was a period of many changes, some such as the two World Wars on a global scale, or the emergence of technology on a more personal scale.
Cold War - The Cold War, was the struggle between two blocks of nations led by the United States and the Union of Soviet Socialist Republics (USSR). Direct military conflict did not occur between the two superpowers, mainly because of mutual fear of nuclear warfare.
Depression - The Great Depression followed the Wall Street Crash, which began on 24 October 1929.
Holocaust - The Nazis' assault on the Jews and other peoples between 1933 and 1945.
Russian Revolution - In 1917 Russia went through two revolutions: 24 to 29 February and 24 to 25 October.