Author Jared Diamond wrote a book on this very subject entitled Collapse where he identified five factors that contribute to the fall of certain civilizations;
1) Climate Change: Natural and man-made climate change has been a threat to civilization for a very long time. The ancient Vikings of Greenland experienced this firsthand when an unexpected cooling of their homeland destroyed their civilization around the 1200’s. Today, global warming is a serious concern.
2) Collapse of Essential Trade Partners: Trade and commerce can greatly impact a nations ability to feed themselves. The ancient Mycenaean of the Mediterranean world are the best example of this phenomenon. Although other factors may have also contributed, disruptions in trade caused by the arrival of the Sea People weakened the empire significantly.
3) Failure to Adapt to Environmental Issues: Man-made environmental changes such as soil depletion, over hunting or over farming were also issues. The Maya are a classic example of how soil depletion can cause collapse. The soil used to grow Maize, their main food source, had been over farmed to the point that it no longer contained enough nutrients to produce. This resulted in a de-stabilization of the rest of the civilization and its eventual collapse.
4) Hostile Neighbors: Countless civilizations have ended by being absorbed or destroyed by more powerful or militaristic societies. The Comanche tribe of the American Central Plains conquered numerous civilizations. The Khmer Empire of Southeast Asia was eventually destroyed by the Ayuthaya civilization.
5) Environmental Problems: Natural changes to a civilizations environment such as volcanic activity, erosion and drought were a major source of collapse for civilizations. The Anasazi of Chaco Canyon disappeared in part due to major droughts.