Whether occupied by friend or enemy, towns and cities were centers of actual or potential wealth and power. They were also consumers of goods and services many of which could not be home-produced. Food must come from herdsmen or farmers. Building materials must come from forests or mines. Such raw materials, the essentials of daily life, must be brought into urban centers when and as wanted. Food and raw materials could be secured occasionally by plunder. A regular supply depended on trade and commerce, or on tribute levied and collected periodically from associated or dependent peoples. In the long run trade and commerce proved to be more reliable and more productive than plunder. As urban centers grew and developed, they established regular channels of trade and communication, by land and water. Along these channels needed imports moved into the urban centers and exports in exchange moved from the urban centers into the back country or the provinces. At every stage in the process care must be taken to prevent intervention by thieves, robbers or envious rivals. Two devices were used to meet this situation: money to facilitate exchange and a defense organization to deal with intruders. Money and its uses developed money changers, money lenders and banks. Bankers and banks exchanged currency at a profit and extended credit. Weapons in the hands of trained personnel evolved into locally employed police and centrally organized armed services, performing police functions and fighting wars, domestic and foreign. Politics, local, regional or national, developed with the growth of population, the profits of expanding urban life, production, technology. As its scope broadened geographically city survival depended increasingly on wealth and power (money and weapons). During periods of peace and stability the civil authorities controlled public affairs. In emergencies, such as natural disasters, invasion, civil or international wars, the military authorities took command. Military authority is an institutional feature of every civilization. In periods of public danger it enjoys complete ascendancy. Like civil authority, the military is a permanent and frequently the dominant feature of each civilization. It is assured of ample income and entrusted with the installations and implements of war making. Both in income and in prestige the military holds a preferred position.