Such empires have generally been characterised by the develop- ment of an extensive cash economy, permitting complex economic exchanges over long distances. These same distances have required efficient means of communication amongst the 'civil servants' of the empire, who must also be capable of working together in a coordinated fashion. The empire can only survive militarily by deploying its military resources over long distances to optimum effect. Thus literacy and bureaucracy as well as good roads (or a navy) and professional soldiers become a necessity.