Operation Dark Winter was the code name for a senior-level bio-terrorist attack simulation conducted from June 22–23, 2001. It was designed to carry out a mock version of a covert and widespread smallpox attack on the United States. Tara O'Toole and Thomas Inglesby of the Johns Hopkins Center for Civilian Biodefense Strategies / Center for Strategic and International Studies, and Randy Larsen and Mark DeMier of Analytic Services were the principal designers, authors, and controllers of the Dark Winter project.
Dark Winter was focused on evaluating the inadequacies of a national emergency response during the use of a biological weapon against the American populace. The exercise was solely intended to establish preventive measures and response strategies by increasing governmental and public awareness of the magnitude and potential of such a threat posed by biological weapons.