Harappan Civilization's writing has never been decoded. Harappan Civilization is a part of the Indus Valley Civilization. The languages of the Harappan Civilization belong to the Bronze Age and they are still today a mystery. This language is totally unattested to any source that is readable. This is one mystery that is yet to be solved.
"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.
