The 1987 stock-market crash that prompted coordinated U.S. circuit breakers was Black Monday.
The worldwide sell-off on 19 October 1987 revealed how rapidly computerized orders and interconnected exchanges could magnify a decline. The Dow Jones Industrial Average lost 22.6% in one day, while other major markets also fell sharply. Regulators investigated market structure, order handling, and the effects of portfolio insurance.
After the crash, U.S. exchanges developed circuit breakers that could temporarily halt trading during exceptionally large moves. The specific thresholds and mechanisms have changed over time, but the basic goal is to give investors and market systems time to absorb information instead of allowing an uncontrolled cascade.
Circuit breakers do not prevent losses or guarantee a recovery. They are pauses in trading. The 2010 Flash Crash later led to additional safeguards, but it was Black Monday that drove the original modern U.S. market-wide response.