Another reason for the Second Amendment

Alien invasion.

He growled, remembering the 35 percent of his original regiment he’d lost subduing what had once been the city of Cincinnati. Division Commander Resuk had gone in with three regiments; he’s come out with less than one, and they’d still ended up taking out over half the city from orbit. Particularly in the nation the humans had called the “United States,” there’d seemed to be more guns than there were people. – David Weber, Out of the Dark. from Warriors

Go get a copy and enjoy.