It is a Precarious Balance

... In primitive times, individual population centers were virtually self-supporting, living on the produce of neighboring farms. Nothing but immediate disaster, a flood or a pestilence or crop failure, could harm them. As the centers grew and technology improved, localized disasters could be overcome by drawing on help from distant centers, but at the cost of making ever larger areas interdependent. In Medieval times [i.e., speaking of the mid-20th Century], the open cities, even the largest, could subsist on food stores and on emergency supplies of all sorts for a week at least. When New York first became a City, it could have lived on itself for a day. Now it cannot do so for an hour. A disaster that would have been uncomfortable ten thousand years ago, merely serious a thousand years ago, and acute a hundred years ago would now be surely fatal.

-- Isaac Asimov, The Caves of Steel (1954).