The first unofficial American Mardi Gras took place on March 3, 1699, when French explorers Pierre Le Moyne D'Iberville and Sieur de Bienville landed near present-day New Orleans, Louisiana. They held a small celebration and dubbed their landing spot Point du Mardi Gras.
The correct answer is the Influenza Pandemic.
This Influenza (flu) Pandemic killed between 20 and 100 million people (estimates differ) - more than the war itself!
The source of the Pandemic is still not known, and people suggest such places as the US, China and France.