When the Roman Empire withdrew from the British Isles in the year 410 it led fairly rapidly to the decay of urban settlements. Even a major city, with an advantageous location like Londinium, could not sustain itself. It was only when Anglo-Saxon Kings began to fortify settlements that urban life revived on any large scale. Extract from the Wikipedia article on Anglo-Saxon London.
All of which supports the idea that you need quite a strong degree of social organisation to sustain an urban culture. If that social organisation disappears the cities crumble.
I am not sure that any American would want the degree of mass destruction that would be needed to de-urbanise North America for a few centuries.
