Why do complex societies collapse?
By John Robb

One of interesting working theories we have is that
while complex societies can be in decay for a long period of time, they only
collapse when its favored elites abandon it/betray it.
The Collapse of The [Western] Roman Empire
One outcome of diminishing returns to complexity is illustrated by the collapse of the Western Roman Empire. As a solar-energy based society which taxed heavily, the empire had little fiscal reserve. When confronted with military crises, Roman Emperors often had to respond by debasing the silver currency (Figure 4.2) and trying to raise new funds.