Because the Brits and Americans came with their guns and kicked the Nazis out?
Why does everyone think they want to line up in “open battle” like redcoats or something? If it ever came to that, it’d be an American insurgency, in which small arms would be quite useful (just ask the Syrians). And yes, I’d expect that if a significant number of Americans felt it justified and necessary to literally fight against the government, that a not-insignificant portion of those Americans would be defectors from the security forces.
Yeah, they did such a good job of disarming Iraq and Afghanistan, I’m sure that would work in the US …