War of Nerves: Urban Guerrilla Warfare

Most armies prefer to avoid urban warfare. Urban campaigns tend to be time-consuming and manpower intensive. They are usually bloody events that burn through manpower, materials and resources at a very high tempo. Most guerrillas are also advised by their strategists to avoid using the urban environment as their battlespace. The cramped and claustrophobic conditions of some cities have created conditions where combatants have entered and emerged with some degree of psychological impairment. However, against the odds, some urban guerrillas have found ways to turn the adverse environment to their advantage. What is the key to their success? How have they managed to turn the "War of Nerves" against the armies sent in to wipe them out? What are the factors and conditions of this type of conflict? Is it really possible for small guerrilla bands to emerge victorious in this sort of conflict?