...and from another thread:

Subject: RE: BS: Previous Presidents and Hurriicanes
From: Bev and Jerry - PM
Date: 25 Sep 05 - 02:30 AM

"The Posse Comitatus Act was passed during the reconstruction era and it makes no mention of a state having to make a request for federal troops. What it does say is that federal troops cannot legally be used for civilian law enforcement. Federal troops can provide equipmnt and supplies, transportation, technical assistance, training and other functions but they cannot be used for law enforcement."

Like Bev and Jerry said, the President should have sent in aid a week before Katrina hit instead of waiting a week...