Katrina was an interesting case scenario. We had federal troops assisting (82nd Airborne for one) and a lot of National Guard troops. Hell, it isn't that well known, but the Mexican Army sent water filtration units there to assist.

While some would argue it is a fine line or even a non-existing line, there is a difference between the National Guard and Active Army troops. But there is a line. For one, the NG are under the authority of the Govenor of the state and can be requested/sent to other states to assist. We call it Title 32 under the U.S. Code for National Guard being paid/authorized under the color of the Governor's authority. NG can be nationalized like for deployments to Iraq and that is called Title 10. At that time, the Soldiers are under the color of the U.S. Army and federal law and no longer under the authority of the Governor.