Well there is "Martial Law" and "Marshal Law". Under Martial Law the military enforces "the laws". Did you know that a US Marshal (or Deputy) can enforce ANY law that exists in any jurisdiction? That means that a US Marshall could in New Orleans (for example) enforce a federal, State, parish/county, or city law in that venue. One big difference --- the Marshal must enfore an enacted law; the military follow the orders they receive, whether there is an enacted law or not.
I appreciate the historical perspective of Posse Comitas as it relates to the Hayes controversy. Injustice against former slaves and other people of color was rampant for nearly a century in the north as well as in the south. (Boston saw some of the worse racism in the country during the 60's.)
The Posse Comitas act, however, is not simply a political expedient of southern bigotry.
The Civil War effectively Federalized the entire South and placed it under the jurisdiction of the military. This was unacceptable under the Constitution. A grave principle was at stake. Should the US military be permitted to suspend the Constitution?
The principle of State succession had been determined by the outcome of the war between the states. Were there to be any limits put on the use of military force against rogue States or fellow countrymen? Posse Comitas ensued from that dilemma.
In the current era FEMA has the authority to enforce Martial Law and to employ the services of the military. This is in keeping with the spirit of Posse Comitas. Civilian authority remains in control of the military.
The President has acted with restraint. The military has been used in LA for search and rescue freeing the State National Guard to act as law enforcement. Bush insisted that the Gaurd be placed under FEMA to ensure coordination. The Governor balked. It was, however, the Governor's utter incompetence to direct the Guard that led to the catastrophe for which the President has received the blame.
As this story unfolds it remains to be seen if the state of Lousiana has the political leadership to rebuild its state with Federal dollars. Federal leadership, for the present, is the only hope.