It will surprise most people but in 2006, a major change in federal law was snuck into the appendices of the omnibus defense budget bill. Reading in part "(1) IN GENERAL- Section 333 of title 10, United States Code, is amended to read as follows:"
That 333 USC 10 is better known as the Posse Comitatus Act, and apparently was intentionally not referenced by name in the amendment. When Bush's friends explained to him that he couldn't send in military aid after hurricane Katrina because of 333 USC 10, someone put this legislation in motion. It allows a President to issue an executive order declaring a domestic emergency, unrelated to the usual Stafford Act declaration of emergency, and use any and all military resources for domestic response.
Previously, there was debate over which services might or might not be allowed to do what jobs, and since the Coast Guard is *not* a military agency except in time of war, they were the primary responders. (The USCG is an "administrative agency" until and unless Congress declares a formal state of war, at which time they are seconded to the Defense Department and become active military.)
FWIW.
So don't be surprised if you see troops, including full military and not National Guard (who are technically active duty militia, sometimes on state duty, but usually seconded to federal military duty) training or deploying for this role.
Slipping the legislation into the back of a six inch thick stack is worrisome, Congress never knew what they were signing off on. But on the other hand, it will allow massive response to the next major disaster.