The number and density of the world's mega-cities is growing rapidly -- particularly now that the majority of the world now lives in urban environments. This section, pulled from "The Coming Urban Terror" points out how their theoretical importance to the 21st Century's, eventually dominant form of warfare:
Cities played a vital defensive role in the last major evolution of conventional state-versus-state warfare. Between the world wars, the refinement of technologies—particularly the combustion engine, when combined with armor—made it possible for armies to move at much higher speeds than in the past, so new methods of warfare emphasized armored motorized maneuver as a way to pierce the opposition’s solid defensive lines and range deep into soft, undefended rear areas. These incursions, the armored thrusts of blitzkrieg, turned an army’s size against itself: even the smallest armored vanguard could easily disrupt the supply of ammunition, fuel, and rations necessary to maintain the huge armies of the twentieth century in the field.
To defend against these thrusts, the theoretician J. F. C. Fuller wrote in the 1930s, cities could be used as anchor or pivot points to engage armored forces in attacks on static positions, bogging down the offensive. Tanks couldn’t move quickly through cities, and if they bypassed them and struck too deeply into enemy territory, their supply lines—in particular, of the gasoline they drank greedily—would become vulnerable. The city, Fuller anticipated, could serve as a vast fortress, requiring the fast new armor to revert to the ancient tactic of the siege. That’s exactly what happened in practice during World War II, when the defenses mounted in Leningrad, Moscow, and Stalingrad played a major role in the Allied victory.
But in the current evolution of warfare, cities are no longer defensive anchors against armored thrusts ranging through the countryside. They have become the main targets of offensive action themselves. Just as the huge militaries of the early twentieth century were vulnerable to supply and communications disruption, cities are now so heavily dependent on a constant flow of services from various centralized systems that even the simplest attacks on those systems can cause massive disruption.
Most of the networks that we rely on for city life—communications, electricity, transportation, water—are overused, interdependent, and extremely complex. They developed organically as what scholars in the emerging field of network science call “scale-free networks,” which contain large hubs with a plethora of connections to smaller and more isolated local clusters. Such networks are economically efficient and resistant to random failure—but they are also extremely vulnerable to intentional disruptions...