I am reading John Yoo's The Powers of War and Peace and came across the reference to Posse Comitatus. (It is not some hip hop Italian music group)
If you believe that the federal government could have done more to help with immediate aftermath of Katrina then you need to understand The Posse Comitatus Act.
The armed forces are the only ones who have the logistical experience to handle the aftermath of Katrina in New Orleans. Under this act the President, as the Commander in Chief cannot deply the armed forces of the United States within the borders of the United States.
Seeing the short comings of this law, congress passed a law which President Bush singed called John Warner Defense Authorization Act. It allows the President to deploy the military in certain circumstatnces within the United States.