of representation and authorized agency. Types of states Charles Tilly distinguished between empires, theocracies, city-states and nation-states. According to Michael Mann, the four persistent types of state activities are: Maintenance of internal order Military defence and aggression Maintenance of communications infrastructure Economic redistribution Josep Colomer distinguished between empires and states in the following way: Empires were vastly larger than states Empires lacked fixed or permanent boundaries whereas a state had fixed boundaries Empires had a "compound of diverse groups and territorial units with asymmetric links with the center" whereas a state had "supreme authority over a territory and population" Empires had multi-level, overlapping jurisdictions whereas a state sought monopoly and homogenization According to Michael Hechter and William Brustein, the modern state was differentiated from "leagues of independent cities, empires, federations held together by loose central control, and theocratic federations" by four characteristics: The modern state sought and achieved territorial expansion and consolidation The modern state achieved unprecedented control over social, economic, and cultural activities within its boundaries The modern state established ruling institutions that were separate from other institutions The ruler of the modern state was far better at monopolizing the means of violence States may be classified by political philosophers as sovereign if they are not dependent on, or subject to any other power or state. Other states are subject to external sovereignty or hegemony where ultimate sovereignty lies in another state. Many states are federated states which participate in a federal union. A federated state is a territorial and constitutional community forming part of a federation. (Compare confederacies or confederations such as Switzerland.) Such states differ from sovereign states in that they have transferred a portion of their sovereign powers to a federal government. One can commonly and sometimes readily (but not necessarily usefully) classify states according to their apparent make-up or focus. The concept of the nation-state, theoretically or ideally co-terminous with a "nation", became very popular by the 20th century in Europe, but occurred rarely elsewhere or at other times. In contrast, some states have sought to make a virtue of their multi-ethnic or multinational character (Habsburg Austria-Hungary, for example, or the Soviet Union), and have emphasised unifying characteristics such as autocracy, monarchical legitimacy, or ideology. Other states, often fascist or authoritarian ones, promoted state-sanctioned notions of racial superiority. Other states may bring ideas of commonality and inclusiveness to the fore: note the res publica of