I don't think the federal government is ALLOWED to put boots on the ground within a state without first getting an official request for assistance from the state. That is why the president declares someplace to be a 'federal disaster area' so that it's now legal for federal aid to arrive which could include federal employees or even military troops. Of course, most governors have at their disposal that state's national guard which they can activate for a state emergency, and which, if a federal disaster is declared, generally then comes under the command of the national military chain of command for the duration of the emergency.
This is sort of like the fact that the Queen of England is NOT allowed to enter the City of London without the consent of the Lord Mayor of London. BTW, the City of London comprises an area which is only about one square mile in size. Parliament and Buckingham Palace are NOT in the City of London, but rather in Westminster which is part of 'Greater' London, but NOT the 'City'.
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