The three theories are warfare between city-state, overpopulation, and drought.
The Louisiana Purchase strongly suggests that the overarching goal of Manifest Destiny was to expand the border of the United States until the western border hit the Pacific Ocean, since this purchase added a huge amount of land to the continental US.
Sparta was ruled by two kings, who ruled until they died or were forced out of office. Athens was ruled by archons, who were elected annually. Thus, because both parts of Athens' government had leaders who were elected, Athens is said to have been the birthplace of democracy. Spartan life was simple.
