So whether this works mostly depends on how brutal your opponents are prepared to be and what price they are prepared to pay. If you have no nukes you can't deter people hellbent on simply genociding you.
Agreed, but that scenario is highly unlikely. Let's say American Civil War 2.0 happens; what's the Union gonna do, nuke us? Are they gonna genocide 200 million people and hope the world just ignores it? Even if they did, how would they then provide for the cities with all the farmers, ranchers, truckers (not that there would be anything left to ship), miners, lumberjacks, etc., dead or imprisoned? The economy would crumble, famine would sweep the country, the US would likely be one of the biggest importers of foreign aid, a full 180 from what it is now.
So yes, as Rogue said, all it would take to overthrow the US government is AR-15s. Nothing more and nothing less.