Thirty-nine years after Black Monday in 1987, Wall Street faces an identical calendar: October 19, 2026, will fall on a Monday—just as it did in 1987.

Monday, October 19, 1987. The Dow Jones loses 22.6% in a single session, a one-day record that still stands. On Friday, October 16, 1987, an Iranian missile strikes a US-flagged tanker off Kuwait. The following Monday, the US Navy destroys Iranian oil platforms. In 2026, the war with Iran is entering its eighth month, and Brent crude touched $110 last Friday.

A story bought on credit

Collective memory has retained that Monday’s trading session in 1987, but forgotten the week that preceded it. Yet the first part of the market to give way was not the stock market as a whole, but stocks sustained by debt. How can investors, each seeking to protect themselves, collectively precipitate the very collapse they are trying to avoid?

On the evening of October 13, 1987, the tax-writing committee of the US House of Representatives introduces legislation removing the tax advantage on borrowing used to finance corporate takeovers. The very next day, shares in potential takeover targets begin to tumble: their prices had anticipated acquisitions financed with debt. From Wednesday to Friday, the market loses more than 10%, its worst three-session stretch in almost half a century.

In 2026, the stock market narrative also rests on debt. The artificial intelligence giants, long accustomed to sitting on mountains of cash, are now borrowing hundreds of billions of dollars to build their data centers. Since September, that link in the chain has been giving way. This week, bonds issued by CoreWeave, which rents out computing power, fell below 80 cents on the dollar, the threshold for distressed debt, while the cost of insuring against its default reached a record high. In Bank of America’s view, the debt of the cloud giants is the most likely source of a systemic credit event. A view shared by the IMF, which has just warned that a correction in AI will trigger widespread deleveraging.

We must be precise, however: it is not the likes of Meta or Amazon that are in danger of imploding. It is the layer below: operators that borrow without generating enough cash to service their debt. In 1987, too, the large corporations were not the first to tremble. The financing structures were.

Fear trades on

Experts speak in unison: a repeat of 1987 is no longer possible, thanks to the circuit breakers introduced in the wake of that very crash. They suspend trading when the index falls 7%, then 13%, and shut the market down for the day once the decline reaches 20%. In theory, we are therefore protected against outright free fall and a cascade of panic. Yet circuit breakers are merely bandages over a structural fracture. They slow the fall without preventing gravity from taking its course. A trading halt does not eliminate sellers: it puts them in a queue. A fall of more than 20% in a single session has therefore become virtually impossible; a 22% decline over two sessions is far from unrealistic. A circuit breaker limits the speed of a decline, not its depth. Were they not triggered four times in eight trading sessions in March 2020?

Above all, the mechanism that turned the decline of 1987 into a free fall has changed its name, not its nature.

Everyone protected, everyone selling

What makes today’s market configuration troubling is not so much asset valuations as the structural overlap between the two eras.

In 1987, an innovation bearing the reassuring label of “portfolio insurance” had won over major institutions. Yet its mechanism proved diabolical: designed to protect against losses by reducing equity exposure, it called for ever more selling as the indices fell. Deployed on a large scale during those fateful days of 1987, it fed the decline, which in turn triggered the next round of selling. The strategy did not, on its own, explain the disaster, which was also fueled by macroeconomic imbalances and urgent demands for liquidity. It nevertheless exposed a formidable weakness: protection built on the ability to sell always depends on the presence of buyers. When too many portfolios follow the same instructions, that condition can disappear at the very moment it becomes vital.

Technology looks radically different in 2026, yet the mechanics of fragility remain the same. Our era is profoundly shaped by artificial intelligence algorithms and marked by an extreme concentration of capital flows in a handful of mega-cap stocks. Nvidia is worth $5.5 trillion, Alphabet $4.3 trillion and Microsoft $4 trillion, while the five largest companies are preparing to pour nearly $800 billion into this investment theme this year. This level of investment, concentrated around a single narrative, now rivals the railroad manias of the nineteenth century for the title of the greatest episode of concentration in global financial history.

In both cases, we were persuaded that technological innovation would eliminate risk, when all it had done was concentrate it—and make it systemic. Abundant only a few months ago, liquidity evaporates at the first hint of macroeconomic stress. Hedge funds (large speculative investment funds) now hold around $13 trillion in gross assets, more than three times their 2013 level. Under normal conditions, they help markets function smoothly. But beware: these same providers of liquidity can swiftly find themselves rushing to sell, even far from where the crisis began.

Must it take a crash to wake us up?

Today, the danger lies less in the calendar than in a formula made famous by the events of 1987. When money becomes expensive, assets bought on credit are the first to give way, before being flattened by the steamroller of automated mechanisms. That same formula is back in play today, in an environment where bonds compete with equities, AI debt is under scrutiny, and the US Federal Reserve has made inflation its priority.

That fateful Monday in 1987 poses a question that is both philosophical and financial. What happens if the market really plunges—and then breaks?

Would it not be fascinating, even dizzying, to find ourselves hoping that a comparable cataclysm might do more than bring an end to a cycle and a market fueled by liquidity—that it might lead to a rediscovery of fundamental value? Throughout economic and financial history, major market upheavals have offered precious moments of revelation. A chance to purge excess, put a price back on risk and redirect capital toward more disciplined uses.

In 1987, the crash did not destroy the American economy, but accelerated the modernization of market infrastructure. The challenge we face in 2026 is not so much a mechanical repeat of 1987 as a convergence of vulnerabilities. May this October 2026 usher in—without a major crash—a period of slower, more fundamental readjustment, in which the excesses of previous years are gradually absorbed and worked off. There would be something beautiful about a new October Monday marking the beginning of an awakening. But that awakening would have to precede the crash, rather than follow it.