How it actually fails
The anatomy of collapse, read backwards — because knowing the failure modes is what lets you build against them.
Collapse is rarely a single event. In the record it is almost always a loss of complexity: an interlinked society reverting to a simpler, more local one, usually over decades, usually with the people surviving and the institutions not.
Societies solve problems by adding complexity: more specialists, more administration, more infrastructure. Each addition solves something real, and each has a maintenance cost that never goes away.
The returns diminish. Early complexity is enormously productive; later complexity is added to manage the problems that earlier complexity created. Eventually maintenance consumes the surplus that made growth possible, and the society has no reserve left for a shock.
This is Joseph Tainter's argument in The Collapse of Complex Societies, and whether or not it explains every case, it identifies the right thing to watch: the fraction of your output that goes to maintaining what you already have. When that approaches everything, a bad harvest becomes a catastrophe.
Build against it: keep slack. A system running at full efficiency has no capacity to absorb anything.
The two trade against each other and the trade is usually made without anyone deciding to make it. Just-in-time supply is efficient and has no buffer. Monoculture is efficient and is one pathogen from failure. A single road, a single power station, a single water source, a single expert — each is the cheap option and each is a single point of failure.
The reason this happens repeatedly is that the cost of fragility is deferred and probabilistic while the saving from efficiency is immediate and certain. Every individual decision is rational; the aggregate is brittle.
Build against it: deliberate redundancy in the things that would be catastrophic to lose — water, seed, food stores, the archive, and the people who hold critical skills. Accept that it looks wasteful, because it is, and that is what it is for.
Soil erosion, deforestation, salinisation, aquifer depletion, fishery collapse. All are slow, all are invisible year to year, and all are effectively irreversible on the timescale of a human life.
The pattern is consistent: the practice is profitable for generations, the degradation is imperceptible annually, and the failure is abrupt when a threshold is crossed. Salinisation from irrigation without drainage did this to some of the earliest agricultural societies on earth; deforestation for charcoal has ended metal industries repeatedly.
Build against it: measure the stocks, not just the flows. Topsoil depth, water table, forest area, fish catch per unit effort. Write the numbers down annually. A trend nobody is measuring is a trend nobody will notice until it has already happened.
The most under-appreciated mechanism, and the one this site is most concerned with. A society can be physically intact and functionally collapsed, because the people who knew how to run it are gone.
It runs in a specific order: first the ability to improve a technology is lost, then the ability to make it, then the ability to repair it, and finally the ability to operate it. Each stage looks survivable from inside. By the time the last stage arrives, recovery requires re-deriving everything from scratch — and the artefacts sitting there, still visible, are no help at all.
Build against it: write things down, teach relentlessly, keep more than one person competent at everything critical, and keep the lower tiers alive on purpose. A society that can still work at tier 2 survives losing tier 5. One that cannot, falls all the way to tier 0.
Most historical collapses are not resource exhaustion in any absolute sense. They are the failure to coordinate the resources that still exist. Civil war, elite competition, loss of legitimacy, breakdown of trade and trust.
The pattern is that stress raises the value of controlling what remains, which raises internal competition, which consumes the capacity that could have addressed the stress. It is a positive feedback loop and it runs fast.
Build against it: legitimate and removable authority, visible fairness in how work and food are distributed, and mechanisms for resolving disputes that everyone accepts before they are needed. This is tier 0, step 12 for a reason — it is the cheapest step on the entire site and it protects everything else.