War is, at its core, organized violence, but different societies engage in different types of warfare. Nomads engage in open warfare, attacking other tribes when they have the potential to be superior and retreating into the open spaces when they lose it. While settled agricultural societies require walls and fortifications, the forces of warfare change and adapt according to societal conditions, and vice versa, as changes in society affect the nature of warfare. The ancient Greeks believed that citizens had a duty to defend their cities. This participation in warfare, in turn, led to greater rights and democracy. By the 19th century, the Industrial Revolution allowed governments to establish and maintain vast armies, larger than anything the world had seen before, but this also created an expectation among these millions of conscripted men that they would have the final say in their societies. Governments were obligated not only to listen to the people but also to provide a range of services from education to unemployment insurance. Today's powerful nation-states, with their centralized governments and organized administrative apparatuses, are the product of centuries of warfare. Memories and the commemoration of past victories and defeats become part of national memory, and nations need their memory if they are to be cohesive. Such centralized governments, in which individuals see themselves as part of a common whole, can wage war on a larger scale and for a longer period because of their organization, their ability to harness the resources of their communities, and their ability to rely on the support of their citizens. The ability to wage war and the development and growth of human society are part of the same story.