Defining Collapse
There are competing understandings and frameworks for what we call 'collapse' in this context. Here, I often prefer to draw from Tainter, 1988:
Collapse, as viewed in the present work, is a political process. It may, and often does, have consequences in such areas as economics, art, and literature, but it is fundamentally a matter of the sociopolitical sphere. A society has collapsed when it displays a rapid, significant loss of an established level of sociopolitical complexity. The term 'established level' is important. To qualify as an instance of collapse a society must have been at, or developing toward, a level of complexity for more than one or two generations.
Why Study It?
Another well phrased fragment from the same source:
... 'It goes without saying that the collapse of ancient civilization is the most outstanding event in its history. . .' Yet beyond scientific interest there is an additional reason: collapse is a topic of the most widespread concern and the highest social significance. The reason why complex societies disintegrate is of vital importance to every member of one, and today that includes nearly the entire world population. Whether or not collapse was the most outstanding event of ancient history, few would care for it to become the most significant event of the present era. Even if one believes that modern societies are less vulnerable to collapse than ancient ones, the possibility that they may not be so remains troubling. In the absence of a systematic, scientific treatment of collapse such concerns range un-tethered to any firm, reliable base.
Complexity
. . .rapid, significant loss of an established level of sociopolitical complexity . . .
Complexity is generally understood to refer to such things as the size of a society, the number and distinctiveness of its parts, the variety of specialized social roles that it incorporates, the number of distinct social personalities present, and the variety of mechanisms for organizing these into a coherent, functioning whole. Augmenting any of these dimensions increases the complexity of a society. Hunter-gatherer societies (by way of illustrating one contrast in complexity) contain no more than a few dozen distinct social personalities, while modern European censuses recognize 10,000 to 20,000 unique occupational roles, and industrial societies may contain overall more than 1,000,000 different kinds of social personalities (McGuire 1983: 115 ).
Tainter explores the topic from a western social science, anthropology point of view. Complexity is studied in an interdisciplinary fashion and social systems are sometimes called a complex adaptive system.
Maximum Power Principle
Complex societies historically tend to dominate simpler societies. There may be a compelling case to be made to similarities in other biological systems pointing toward the reasons for this:
The maximum power principle or Lotka's principle has been proposed as the fourth principle of energetics in open system thermodynamics, where an example of an open system is a biological cell. According to Howard T. Odum, "The maximum power principle can be stated: During self-organization, system designs develop and prevail that maximize power intake, energy transformation, and those uses that reinforce production and efficiency."
Energy
Our rising complexity has given us an incredible energy subsidy, primarily and most recently in the form of fossil fuels, that we use to fuel our own growth. This extraction and exploitation feeds a relative social comparative advantage, at a steep and rising cost of unaccounted externalities. Several common examples of these external factors being anthropogenic forcing in climate change dynamics, resource depletion, and ecological destruction from continued and unrelenting human system development. Energy is at the root of my own personal framework of understanding these systems as a layperson. This is one lens of many we can use to explore an incredibly diverse topic together.