[microcolonel]

> Why do walls marking national boundaries proliferate amid widespread proclamations of global connectedness and despite anticipation of a world without borders? Why are barricades built of concrete, steel, and barbed wire when threats to the nation today are so often miniaturized, vaporous, clandestine, dispersed, or networked?

Because some specific threats, specifically to sovereignty, can be deterred, retarded, and/or prevented by a wall.