On September 11, 2001, at 8:46 a.m., when there were 43 minutes left until the New York Stock Exchange opened, the first plane hit the North Tower and trading was canceled immediately. Nasdaq followed the same path when the second Boeing struck the South Tower. The American exchanges remained closed until September 17, the longest shutdown since the Great Depression. When they reopened, the Dow Jones fell 685 points in a single day, a drop of 7.13%, the worst daily decline ever at the time, accumulating a loss of 14.26% that week. The S&P 500 retreated 11.6% and Nasdaq lost 16%, a total of approximately $1.4 trillion in market value lost.
The Federal Reserve responded with an accommodative monetary policy that would become a reference for future crises. At an emergency meeting on September 17, it cut rates by 50 basis points to 3% and promised unlimited liquidity. The rate-cutting cycle continued until rates reached 1% in June 2003, the lowest since 1958. This rapid response, with an explicit promise of liquidity, was repeated in 2008 and during the 2020 pandemic.
No sector was more transformed than civil aviation. The FAA closed all American airspace and September traffic fell more than 30% compared to the previous year. The United States created the Transportation Security Administration in November 2001, reinforced cabin doors appeared, stricter baggage screening and passenger watchlists were introduced. The insurance sector faced approximately $60 billion in losses, leading to the creation of public guarantees for terrorist acts and the growth of the catastrophe bonds market. The European Union also reinforced supervision, with new regulations on digital operational resilience in force since January 2025.
The legislative response came with the Patriot Act, passed on October 26, 2001, which strengthened customer identification and suspicious transaction reporting mechanisms, impacting financial institutions worldwide, including in Portugal. Global military spending reached $2.887 trillion in 2025, the 11th consecutive year of increase. Twenty-five years later, the most enduring legacy of September 11 is not in the stock markets, which recovered quickly, but in the way central banks, insurers
