Which factor led to the rise of most Mesopotamian empires?

This question has already been asked on , so here is the link :
Your answer would be The rise of a young all-powerful ruler

One of the things that allowed native americans in mexico to settle cities larger than those in areas to the north was "b. the development of cities that were built by an earlier civilizations," since mexico had been the home of some of the world's most advanced civilizations before the columbian invasion