Why? Why the police, any more than the military?
And you still have yet to explain your “invaders” comment - are you saying you consider legal immigrants (permanent residents, even) to be “invaders”?
The military should only be citizens too.
We are so fractured and balkanized already with thirty million non english speakers refusing to assimilate. We should have a twenty year moratorium on all imigration.
Citizenship should be hard to get and mean something.