I think this is a decent summary of the most important challenge to anarchist theory:
"virtually the entire globe is now already occupied by states. Throughout recorded history, small autonomous groups of people have been extraordinarily vulnerable to conquest and absorption by larger states, a phenomenon that continues to the present day. Thus either the return to a life of small autonomous stateless groups would have to occur almost simultaneously throughout the world or some states would continue to exist with their exceptional capacity and propensity for conquest and absorption. If anarchism requires the first, then anarchism must be set aside as at best an appealing fantasy. If it does not, then it must show why states would permit any small, independent group to exist anywhere on earth, with the possible exception of a few of the most remote and forbidding places on the globe where almost no one, and probably few advocates of anarchism, would care to live." Robert A. Dahl, Democracy and Its Critics at 47.