An empire is a large, multi-ethnic state, whose political structure is held together by Coercion. (Compare with a Federation, where a large multi-ethnic state is based on mutual agreement between the participants.)... Historically, most empires came into being as the result of a militarily strong state conquering other states and incorporating them into a larger political union. Often these multi-ethnic entities were strengthened by the introduction of a common religion. http://www.wikipedia.org/wiki/Empire
Even more likely to lead to the Collapse Of Complex Societies.