Originally it was a Founding Fathers argument: that an armed citizenry is the only sure protection against an out-of-control government.
That has never stopped the government, by the way, from putting down the various armed rebellions that have broken out during our history, including a pretty big one in 1861, but that was in fact what the founders believed, and it's become a big part of our national psychology.
And actually, considering how many people believe the Founding Fathers argument, I'm surprised America isn't more heavily armed than it is. If the idea is to prevent tyranny, you could argue the citizenry should be able to match the army - gun for gun, grenade launcher for grenade launcher. Drone for drone.
It's remarkable that the gun lobby has pretty much conceded that we only have a right to own guns, nothing more.
But now - look at the other argument that's taking center stage. Even as the NRA called for putting armed cops in every school, Senator Lindsey Graham was warning police departments to expect LESS federal money for cops, not more, and then went on to say that because it might take longer to get a cop, well, that's why he himself has an AR-15.
"I would be better off protecting my business or my family if there was a law and order break down in my community - people roaming around my neighborhood - to have the AR-15 and I don't think that makes me an unreasonable person."
Right - So that when a weakened government lets the social order break down, he can protect his family. Which is the exact opposite - of the original argument!
So I guess the way this would work is, we the citizenry arm ourselves to fend off the government when it gets too strong, and then, once we've emasculated the tyrants, we turn the guns on each other to protect ourselves from the social collapse that follows.
Either way, we are going to need a lot of guns.
Apparently , somewhere along the line, we became Libya.