There is statistically insignificant risk.
The US shares land borders with only two foreign countries: Canada and Mexico. We have been at peace with the former since 1815, and strong allies since 1940. Relations with the latter have been a bit less cordial, exacerbated by racism and corruption. But there's very little risk of invasion there. So any potential direct attacker would have to come by sea, through the largest navy on Earth.
I think the only way to reasonably do it would be by subverting an entire municipal government and then buying up military hardware through the police department. The force would have to be trained to capture and use military assets already in place. But then what would they do with them? Nothing you could do would be more profitable than just sitting back and running the government you already control just like a corrupt American politician would. And the reason is that suburban Americans would NIMBY a foreign invasion just as strenuously as they would a proposed nuclear power plant, a new Wal-Mart, or some mid-rise apartment blocks. Just think how that go if everyone were allowed to shoot their guns at each other.
Most people do not own firearms. But those who don't mostly live in cities. Everyone in a rural area owns at least one, without exception (hyperbole), even if it's just the one they use for unsalvageable injured livestock and large predators. Suburbanites keep rifles for recreational target shooting and hunting, but for personal defense you're probably better off with a shotgun or a sidearm. A lot of my neighbors carry daily, for no particularly good reason that I can discern. I don't care to know who's packing and who isn't, but you can often tell by looking at the purses or clothing.
I don't have one, but I saw a neighbor shoot a snake on the sidewalk across the street once. That guy's kind of an idiot, but he didn't miss. It wasn't venomous, and he probably didn't eat it, but he didn't have to be afraid of it any more, either. So I guess it wards off the bogeyman?
I'd estimate that 98.6% of rural households own at least one gun, 20% of urban households keep some, and 50% of suburban households. Most people who own any guns at all have no more than one sidearm, one rifle, and one shotgun, and a large number of the total number of guns are found in collections of 20 or more. You have to be rich or inherit antiques to hoard guns like that. Most people would probably stop after a 1911A pistol or .357mag revolver, a Mossberg 500 or Remington 870 or similar 12ga pump shotgun, and a .22LR rifle. Those guns are cheap, versatile, use cheap and widely available ammunition, have plenty of clones, and can serve most gun-owner purposes that's aren't hunting things larger than squirrels. A subset will also get a 5.56 or .223 military-inspired rifle for keeping the king of England out of your face, and a mostly disjoint subset will have one very good larger-caliber rifle and huge chest freezer for hunting deer every November. Hunters and hoplophiles tolerate one another, but they are usually more allies than friends.