My guess is that it would take a "Presidential Finding" followed by and "Executive Order", possibly backed up by a "Congressional Resolution".
Bear in mind that it doesn't take a "Declaration of War" by Congress as prescribed by the Constitution for the President to take the nation to war anymore.
If asked by the Governor or the legislature the President could enact the "Insurrection Act" and send in the armed forces to keep the peace. If that is not done, then armed forces are prevented from "enforcing state laws" by posse comitatus.
NG on the other hand, are under the command of the Governor of the State and they along with local police and state police can enforce law.
I heard Blanco say today that law officers coming from other states would be deputized in LA, therefore making them able to enforce the law.