Let's take a look at just raw numbers. The entire United States military (including clerks, nurses, generals, cooks, etc) is 1.2 million. Law enforcement is estimated at about 1.1 million (again, including clerks and other non-officers.) This gives us a combined force of 2.3 million people who could potentially be tapped to deal with a civil insurrection. Keep in mind this also includes officers who serve in the prisons, schools, and other public safety positions that require their presence. That total of soldiers is also including US soldiers deployed to the dozens of overseas US bases in places like South Korea, Japan, Germany, etc. Many of those forces are considered vital and can't be removed due to strategic concerns.
But, for the sake of argument, let's assume that the state slaps a rifle in every filing clerk's hand and tells them to sort the situation out.
We now have to contend with the fact that many law enforcement and military personnel consider themselves patriots and wouldn't necessarily just automatically side with the state if something were to happen. There is a very broad swath of people involved in these communities that have crossover with militia groups and other bodies that are, at best, not 100% in support of the government. Exact numbers are hard to pin down but suffice it to say that not everybody would be willing to snap-to if an insurrection kicked off. Even if they didn't outright switch sides there's the very real possibility that they could, in direct or indirect ways, work against their employer's prosecution of the counter-insurgency either by directly sabotaging operations or just not putting as much effort into their work and turning a blind eye to things.
But, again, for the sake of argument, let's assume that you've somehow managed to talk every single member of the military and law enforcement services into being 100% committed to rooting out the rebel scum.
There are an estimated 400 million firearms in the US. Even if we just ignore 300 million firearms available as maybe they're antiques or not in a condition to be used, that's still 100 million firearms that citizens can pick up and use. Let's go even further than that and say of that 100, there are only about 20 million firearms that are both desirable and useful in an insurgency context and not say .22's or double barrelled shotguns.
It should be noted just for the sake of interest that several million AR-15's are manufactured every year and have been since 2004 when the "assault weapons" ban ended. Soooo 2-5 million per year for 15 years....
If only 2% of the US population decided "Fuck it, let's dance!" and rose up, that's about 6.5 million people. You're already outnumbering all law enforcement and the military almost 3 to 1. And you have enough weapons to arm them almost four times over. There are millions of tons of ammunition held in private hands and the materials to make ammunition are readily available online even before you start talking about reloading through scrounging.
So you have a well equipped armed force that outnumbers the standing military and law enforcement capabilities of the country by a significant margin.
"But the military has tanks, planes, and satellites!"
That they do however it's worth noting that the majority of the capabilities of our armed forces are centered around engaging another state in a war. That means another entity that also has tanks, planes, and satellites. That is where the majority of our warfighting capabilities are centered because that's what conflict has consisted of for most of the 20th century.
We've learned a lot about asymmetric warfare since our time in Iraq and Afghanistan and one of the key takeaways has been just having tanks and battleships is not enough to win against even a much smaller and more poorly armed opponent.
A battleship or a bomber is great if you're going after targets that you don't particularly care about but they don't do you a whole hell of a lot of good when your targets are in an urban setting mixed in with people that you, the commander, are accountable to.
Flattening a city block is fine in Overthereastan because you can shrug and call the sixty civilians you killed "collateral damage" and no one gives a shit. If you do that here, you seriously damage perceptions about you among the civilians who then are going to get upset with you. Maybe they manage to bring enough political pressure on you to get you ousted, maybe they start helping the rebels, or maybe they pick up guns of their own and join in. You killed fifteen fighters in that strike but in so doing you may have created thirty more.
Even drones are of mixed utility in that circumstance. It's also worth noting that the US is several orders of magnitude larger than the areas that drones have typically operated in during conflict in the Middle East. And lest we forget, these drones are not exactly immune from attacks. There's also not a lot a drone can do in places with large amounts of tree cover...like over a billion acres of the US.
And then even if we decide that it's worth employing things like Hellfire missiles and cluster bombs, it should be noted that a strategy of "bomb the shit out of them" didn't work in over a decade in the Middle East. Most of the insurgent networks in the region that were there when the war started are still there and still operating, even if their influence is diminished they are still able to strike targets.
Just being able to bomb the shit out of someone doesn't guarantee that you'll be able to win in a conflict against them.
Information warfare capabilities also don't guarantee success. There are always workarounds and methods that are resistant to interception and don't require a high level of technical sophistication. Many commercial solutions can readily be used or modified to put a communications infrastructure in place that is beyond the reach of law enforcement or the military to have reliable access to. Again, there are dozens of non-state armed groups that are proving this on a daily basis.
You also have to keep in mind the psychological factor. Most soldiers are ok with operating in foreign countries where they can justify being aggressive towards the local population; they're over here, my people are back home. It's a lot harder to digest rolling down the streets of cities in your own country and pointing guns at people you may even know.
What do you do as a police officer or soldier when you read that soldiers opened fire into a crowd of people in your home town and killed 15? What do you do when you've been ordered to break down the door of a neighbor that you've known your whole life and arrest them or search their home? What do you do if you find out a member of your own family has been working with the insurgency and you have a professional responsibility to turn them in even knowing they face, at best, a long prison sentence and at worst potential execution? What do you do when your friends, family, and community start shunning you as a symbol of a system that they're starting to see more and more as oppressive and unjust?
"People couldn't organize on that scale!"
This is generally true. Even with the networked communications technologies that we have it's likely ideological and methodological differences would prevent a mass army of a million or more from acting in concert.
In many ways, that's part of what would make an insurrection difficult to deal with. Atomized groups of people, some as small as five or six, would be a nightmare to deal with because you have to take each group of fighters on its own. A large network can be brought down by attacking its control nodes, communication channels, and key figures.
Hundreds of small groups made up of five to twenty people all acting on their own initiative with different goals, values, and methods of operation is a completely different scenario and a logistical nightmare. It's a game of whack-a-mole with ten thousand holes and one hammer. Lack of coordination means even if you manage to destroy, infiltrate, or otherwise compromise one group you have at best removed a dozen fighters from the map. Attacks would be random and spontaneous, giving you little to no warning and no ability to effectively preempt an attack.
Negotiation isn't really an option either. Deals you cut with one group won't necessarily be honored by another and while you can leverage and create rivalries between the groups to a certain extent you can only do this by acknowledging some level of control and legitimacy that they possess. You have to give them some kind of legitimacy if you want to talk to them, the very act of talking says "You are worth talking to." And there are hundreds, if not thousands, of these groups.
You are, in effect, trying to herd cats who not only have no interest in listening to you but are actively dedicated to frustrating your efforts and who greatly outnumber you in an environment that prevents the use of the tools that your resources are optimized to employ.
Would it be bad? Definitely. Casualties would be extremely high on all sides. That's not a scenario I would ever want to see play out. It would be a long, drawn out war of attrition that the actual US government couldn't effectively win. Think about the Syrian Civil War or The Troubles in Northern Ireland or the Soviet-Afghan War in Afghanistan. That's what it would be.