On why I think that large cities could become unsustainable.

Large cities are a more efficient (smaller ecological footprint) settlement structure -- provided that

1. there is cheap transport of resources, above all agricultural products;
   
2. there is efficient organisation that is stable (be it a government or the anarchists' self-organizing).

Now if (a) oil becomes scarce, (b) transport as now structured will become more expensive, (c) the current petrochemical industrial agriculture will collapse, leading to food scarcity. (It doesn't matter whether 5% or 50% short of demand.)

A further consequence of (b) would be a reduction in organisation efficiency and stability. Further consequences of (a) (and (c)) would be an effort by those with power to secure the scarce resources for themselves. This on one hand could mean government rationing while the military is kept well supplied. On the other hand: robbery. Robbery, both for fuel and food, and suppliers evaluating the risk of robbery, would further limit food transport from afar into cities.

In short, I think citydwellers would be compelled to move closer to the sources of food. Even if the new, more rural settlement structure is able to feed less, even if bands of robbers roam the countryside. (They'd roam the cities, too.)

I think prior collapses of civilisations, most notably that of the (Western) Roman Empire and the Classic Maya (Tikal vs. Calacmul) give an example. But there is also a more recent occurence that I think is comparable. Look at this photo:

This is a Hamsterzug (="hamster train"), leaving Hamburg main station in 1946. Whence the name?

At the end of WWII, in Europe's bombed-out cities, urban food scarcity became a reality. What remained of transport infrastructure capacity was mostly used by the military. The nodes of the processing/distribution network were damaged, too, including slaughterhouses, mills, and shops themselves. There was widespread robbery, too.

So the lucky could temporarily move in with rural relatives (as some of my ancestors did). The less lucky travelled to the countryside and tried to 'buy' food from the peasants by giving them their valuables. But if nothing remained for sale, one thing remained: going into the woods to hunt for -- hamsters.

_{*Lunatic*, n.
One whose delusions are out of fashion.}