September 11th rebuilt the airline industry. Delta is the result.

The anniversary renews the grief; the markets mark something else. Twenty-five years ago this week, hijacked jets did not merely kill thousands of people. They bankrupted an industry, then forced the state to rebuild it in a form that could survive. Of the economic legacies of September 11th, few are as durable as this one: the fragmented, cheerfully loss-making airline business was consolidated into a disciplined oligopoly. Delta Air Lines, once a casualty of the shock, is now its most profitable expression.

Start with the world that died. For two decades before 2001, the deregulated airline industry had been a graveyard of margin: a dozen legacy carriers fought over seats, cut fares at the margin and, in aggregate, repeatedly failed to earn their cost of capital. The attacks looked like the coup de grâce. Planes were grounded for days, demand collapsed, and the carriers faced a winter of ruin precisely when they could least afford one.

The political response, delivered with remarkable speed, rewired the economics. The Air Transportation Safety and System Stabilization Act, signed on September 22nd 2001, made up to $5bn in direct payments available and added $10bn in loan guarantees. Weeks later the newly created Transportation Security Administration federalised airport screening, lifting the cost of security off the airlines' balance sheets — though passengers now pay a $5.60 federal security fee on every one-way ticket to fund it. In effect the state did two things at once: it floated the industry through its worst month, and it socialised one of its biggest fixed costs.

The deeper change was structural, and it took a decade to complete. Bankruptcy did the reorganising that markets had failed to force: Delta itself entered Chapter 11 in 2005 and emerged in 2007, having shed contracts and debt. Then came the mergers — Delta and Northwest in 2008, United and Continental in 2010, American and US Airways in 2013 — until a handful of carriers, the "big four", controlled about three-quarters of the American market. Fragmented competition gave way to oligopoly, and oligopoly discovered the discipline that competition never permitted: they stopped flogging capacity, stopped underpricing and finally charged for the product.

The payoff is visible in Delta's present. In the three months to June it booked record revenue of $17.7bn, up 14% on the year, with pre-tax profit of $1.4bn and a double-digit return on invested capital; it affirmed full-year adjusted earnings of $6.50–7.50 a share. The aggregate signals of AInvest, a ratings service, label the stock a Buy. This is exactly what a survivor of a shock looks like when the shock's aftermath hands it pricing power.

Here is the trouble. For a company growing on this scale, the valuation is strikingly unambitious: a trailing price-to-earnings ratio near 13 and a forward multiple in single digits. Investors are not paying for the premium business; they are paying a cyclicals' price, because they doubt the rents will last. The doubt deserves to be taken seriously, because the oligopoly is a behaviour, not a law.

Capacity discipline is a decision that any carrier can reverse, and it typically cracks when costs rise and growth stalls. This year's jet-fuel surge has already pushed Delta to "meaningfully" cut its growth plans and to raise checked-bag fees — the reflex of an industry reaching for the same lever, higher fares, precisely when its pricing power is being tested. And the $5.60 security fee sits as a standing tax on every ticket, a reminder that the state's help, however welcome in 2001, permanently took a cut of the demand it helped preserve.

The anniversary's real lesson is general, about what shocks do to structure. A catastrophe, followed by state money and forced consolidation, can turn a swamp of competitors into a disciplined few who finally cover their costs. Delta shows the gains that structure produces. Its low multiple shows what the market fears about them — that the discipline is a choice, and that choices can be reversed in the next downturn. Ten years before its bankruptcy it was a loss-maker; now it is priced as one that cannot be trusted to keep making money. That tension, not the memorial service, is what a shareholder should think about on the twenty-fifth anniversary.