During the early 1920, the U.S rejected to join the League of Nations (that the President Woodrow himself proposed), high tariffs imposed by the Fordney-M Cumber act prevented international trade, and the emergency quota act created barriers for immigration from some countries. All these actions isolated America from International Affairs.
"Sumerian city-states fought each other" would be the best option from the list, since this internal division led to a collapse of power that left people without protection or a solid economy.
