Have you ever held a rifle?
Fired a rifle?
Fired it at long range?
Do you think that being an MP somehow makes a person magically immune to bullets?
What do you think happens to a military base when it gets cut off from power, water, and sewage, and demands are issued that the people involved in atrocities be turned over?
What do you think happens when expeditionary forces sent out to forcibly reconnect the utilities get ambushed, firebombed, etc, etc?
Almost the entirety of your position is based on 'What one side does will be effective, what the other side does will not be.' That's not how war works. That's especially not how civil wars work.
'I have orders to defend this base' is a lot less of a motivation than 'my family was killed by a psychotic mass-murderer, I want revenge, and I have nothing left to lose.'
The armed forces in the USA have 1,328,000 people in uniform. The reserves contain about another 800,000. That's about 2.1 million people in total. Of those, only a fraction are 'combat arms,' and the rest are involved in a whole host of support roles. The military as a whole is outnumbered about 150:1 by the civilian populace, and when you shave away the parts that would either not side with leftist federalists in the first place, or would immediately refuse to obey any further after a nuke is dropped, those odds scale much, much higher.
If we take unrealistically pessimistic assumptions, assume the whole military *does* remain with the federalists, and divide the populace into 20% left, 20% right, 60% undecided, we're left with 30:1 numerical advantages to the civilians, or 63 million vs 2.1 million. Now let's assume that only one in a hundred of those 63 million are actually willing to go do something stupidly dangerous to punish those responsible for nuking.
That gets you 630,000 people willing to die to get some revenge. If only one hundredth of those are close enough to meaningfully act against whatever Dems are in DC at the time, that's still 6300 partisans making everyday life hell, and being out in the open a lethal liability for them.
This isn't a 'regular' threat out in the open that you can easily see and engage. This is someone parking their car ten blocks down the street, using it to create a blind, and then waiting with a rifle for a shot. It's someone loading a car with explosives, and driving it into the gate of your military base or government structure. It's poisoning the water supply, demolishing power substations and lines, it's a million things that are hard to stop, and even harder to know are coming until after they happen.
Yeah, a lot of these insurgents/partisans would die, but a lot of the people they're attacking are going to die too. And the soldiers, the police, and everyone else who is getting killed protecting the people who pushed the button?
They have 'following orders' for a motivation. Very, very few people are actually fully onboard with the leftist ideology, and most of them are only on board with it for as long as it doesn't cost them personally. Once the bodies start falling, they're going to be asking themselves 'Am I really willing to die for this?'
And on the other side, you have people who know that it is literally do or die. Surrender is not an option, because if the enemy gets their hands on you, you'll be lucky if they just kill you, more likely they'll rape you first, and tell you about how righteous they are for doing so.
Two things matter in war, the capacity to fight, and the will to fight. There are about 300 million too many guns in the US for the capacity to be able to be destroyed, especially as home manufacture of new guns and ammunition is very much a thing.
Once a nuke is dropped, the will to fight becomes functionally infinite, for the same reason that the Ukrainians are fighting like hell against the Russians right now.