ON SATURDAY, AUGUST 27, 2005, the mayor of New Orleans called on the people of his city to evacuate voluntarily ahead of Hurricane Katrina. The governor of Louisiana and the president of the United States had already declared a state of emergency, and by midday on Sunday, the mayor made his evacuation order mandatory. When the storm hit shortly before dawn on Monday, however, many people remained behind, some by choice and some because they had no choice.