As I recall, if you are a soldier and follow an unlawful order, you and your superior who gave the order, are responsible for that action. It is your responsibility to refuse the order and report it up your chain of command.
I did a lot of mental wargames with ex-military people over the years. They are interesting to think about. I wouldn't want to be anywhere near it if it happened.
This is just a frontal assault scenario, extremely simplified. It also assumes the rebel faction has sufficient numbers to make it past the first encounter.
Imagine the first wave is 100 rebels storming the White House. The Secret Service and local law enforcement will be calling for a cleanup of 100 bodies on the lawn. Possibly the arrest of a few injured survivors.
You can be sure reinforcements will be called in.
The second wave of 1,000 rebels attempt to storm the White House. They might make it to the newly extended buffer zone, blocks away.
As someone else said, these civilians with weapons and passenger vehicles are facing the military at this point. They aren't just going to hang out waiting at the perimeter. They're going to aggressively hunt and stop the threat. So word will spread when that hunting goes a bit wrong. Like a AH-64 lights up a suspected rebel faction stronghold with the M230. That may turn out to just be an apartment building with women and children in it. That could have even been
Around the 4th or 5th attack, assuming the rebel propaganda machine was working, soldiers are going to start questioning what they're doing. They might sympathize with the rebels. They might recognize the fact that they're fighting other Americans. Who is right, a bunch of people willing to die over their belief, or the people who sign the paychecks. This is when the power may shift. The military would still have superior communications. Rebel factions would have poorer communications and intelligence, but would have numbers and motivation.
BTW, I intentionally wrote that with no motives behind it. It really doesn't matter what the reasons are. It will be messy.
There are other tactics that would change things dramatically. Say it were done quieter and a single shift at a single air base was infiltrated. They could put a few squadrons of fighter/attack aircraft in the air and on target in less than an hour. It could be booked as an "exercise". There is one I found with just a few quick searches that has enough aircraft and weapons to take a small country. I'm sure there are others.
pnutjam said in another comment, that won't happen and to use Syria as an example.
ISIS did that in Syria. Al-Tabqa air base, Aug 2014. That was less than 1,000 ISIS fighters in 3(?) waves.
I read mixed reports on how many aircraft they captured, how many were functional, and if they were used. Some said none were functional. Another report said two ISIS MiG-21Bs were shot down there. Other reports say that they are still using some MiG-21Bs on ground strikes.
So, yes, civilians with any sort of weapon can be dangerous. It doesn't even have to be homemade AR-15s.
Luckily, American civilians are disorganized and poorly motivated. Hell, look at OWS (New York). They had up to about 50,000 marchers. That's not a little protest. That's an army. They had no goal and no coordination, since they were intentionally lacking leadership. It could have flipped from being unarmed, to capturing everything the police brought. But even if they did that, they wouldn't have a clue where to go after that.