Attrition. After lack of food, water and medicine starts killing cement dwellers, residents will start killing each other. Cops and the National Guard will kill many more. When the food is completely gone cement dwellers will head for the burbs first. Lots of guns in the burbs and guns will be used to reduce marauders even more. Small towns more than 50 miles from the cities will be next. These areas will have a somewhat organized resistance as they will have plenty of warning. Last to be hit will be farming towns far away from cities. These places will be completely organized, have lots of heavy equipment, and will be fighting for their families. Residents of these far away farming towns will know the lay of the land and disorganized food thieves will be cannon fodder. Weather in the northern states will kill many of the marauders. I do not see food thieves even getting into farm country. They will kill each other and their numbers will be reduced to nothing. Imagine gang bangers attempting to take a farming town that is armed to the teeth and willing to die for their families. Bye bye cement dwellers.