The Oversoul immediately decided that ambushing any human forces that attempt to leave the cities would have a much more powerful effect than committing enough forces to assist. They continue to assist in identifying the other taints, though most of their efforts were directed to destroying attempts by the humans to relieve forces. The air walkers were indispensable, using their ability to function like submarines in a water environment to destroy boats. Food, civilians, and  trade boats and caravans would make it through fine, but if there were military units or goods, those convoys would inexplicably vanish, and mixed conveys would report the ease and precision of the destruction of military parts, while the attackers themselves never showed themselves. The effect of these phantom attacks along with the fact each city has become a fortress, rather than part of a network, would shatter human moral, especially when the 'destroyed' military supplies started showing up in elven hands.