Well, thanks to the Insurrection Act and the Posse Comitatus Act, the US government is only allowed to use its military on US soil in fairly limited circumstances. Basically those are where there is (or importantly - the President considers there is) insurrection, domestic violence or conspiracy that undermines the execution or enforcement of US law and that the local government is unable or unwilling to deal with the situation.
That rules out the Army (and the MPs are there to police the military not civilians in any case). National Guard would work but only if one can say that Puerto Rico’s own government isn’t able to deal with the situation (and Puerto Rico could use its own National Guard if it has one).
Since there doesn’t seem to be much of insurrection, domestic violence or conspiracy at the moment, federal forces are a no-go.
Guess when and why the Posse Comitatus Act was passed?
If you guess after the American Civil War in order to prevent the future imposition of martial law over parts of the US and that it was advocated for by those trying end Reconstruction and to roll back rights for former slaves, you’d guess correctly.
Back then that was the Democrats by the way. Aren’t history and politics fun?
