Which 1873 event is widely associated with the start of the Panic of 1873 in the United States?

The failure of Jay Cooke and Company on September 18, 1873, is widely associated with the start of the Panic of 1873 in the United States.

Jay Cooke and Company was a major investment bank that had financed railroads, especially the Northern Pacific Railway. When investors lost confidence in the railway's prospects, the firm could not sell enough bonds and suspended payments. The New York Stock Exchange closed temporarily, and financial stress spread through banks and railroad companies.

The panic was part of a wider international downturn sometimes called the Long Depression. The crisis affected Europe as well as North America, and falling railroad investment contributed to prolonged economic weakness. The event is not identical to the 1929 Wall Street Crash: the 1873 panic was closely connected to railroad speculation, bank failures, and the collapse of a major investment house.