As a kind of footnote to my post about the division of responsibility in preventing and dealing with disasters like Hurricane Katrina and the aftermath, including the lack of safety and civic order in New Orleans:
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.Article IV, Section 4 of the United States Constitution.