All of those who serve are sworn to defend the constitution, not an ideology or particular leader. All are schooled in what it means to follow ‘lawful’ orders, and the consequences of following ‘illegal’ orders. If one enters the service under false pretense, thats pretty hard to protect against, but no one is supposed to be a robot following orders blindly, particularly when moral questions arise. Rounding up/disarming/shooting lawful citizens would be illegal orders and will quickly lead to mass defections of NG who are by and large of the people.
Frankly, I think civil war is unlikely, though if we swing back to a socialist leadership with two tiered justice and putting the boot on the neck of the working class, civil disobedience would probably be the first major response of a society in collapse. Obviously people will take sides, and in doing so, take their chances. But I dont think those battles would be between citizen and military or national guard. It will be between the lawful and the lawless gimmidats.
Your response mirrors what the frog thinks as he swims in the pot of water. The frog totally ignores the rising temperature of the water as it rises. There are many indicators of the rising temperature; sanctuary cities, violent protests at otherwise peaceful events, violence against police, etc. Look back 20 years, would these things be happening? The stew is getting hotter.