What will happen?
Very hard to say. It will go one of two ways. Either folks will take the traditional American attitude:
I have very mixed feelings about that, because the LE/Military community will be pitted against the citizenry. A lot of the military and LE folks will go AWOL, but not nearly enough, and things will go sideways in a hurry.
Or the same thing will happen that happened in Australia. The hardware stores will sell out of the largest diameter PVC pipes and PVC end-caps almost over night, and millions of guns will be sealed and buried in backyards all over the country.
I have very mixed feelings about that too, because the idea of the American people acquiescing to that level of tyranny seems untenable.
