That and allowing invaders to form their own armies by allowing them to legally own guns.
Military and combat-experienced among them will find ways to convert civilian AKs to full auto. Bomb making materials are available over the counter and from feed supply houses and retailers.
You are already invaded by a foreign force, and the American Quislings who have volunteered to run surveillance on you for it. See my tagline. Until you deal with them, the migrants are the least of your problems.