Army (3500 BC): walled city -> Navy (1210 BC): walled city -> Air Force (early 20th century): walls provide no security; in fact, institutional and physical infrastructure concentration reduces security rather than increases it -> Cybersecurity (early 21st century): decentralized security essential as centralization runs the risk of total destruction. Will the trend of decentralization spurred by the advances of technologies like the blockchain, for example, speed up the process of de-urbanization in the developed world? Can technology also start to reverse the developing world urbanization?