You dumb shit, without immigration our population will shrink. Rapidly.
And age. After 50 years, or even 20, you will have a collapsing society that can't pay pensions, can't pay for medical care, can't grow enough to feed ourselves or make enough for any other need.
Then good luck with your violent fantasies, as the Vatican's Swiss Guards will be more formidable (and maybe more numerous) than the U.S. Geriatric Army.