This isn't really a given. Historically, whenever you have a civil war the existing state's military splits down the middle, with people generally unwilling to fire on friends, family, and neighbors. Former military officers usually form the core of the rebel military, taking their training, experience, and oftentimes equipment with them to fight for the other side.
The mistake here is thinking of the U.S. government as a monolith. Ultimately it's all just people, bound together by being paid for in dollars that are either raised as taxes or borrowed as treasuries. GP's post posits a world where the dollar is worthless; what's binding them together then?
The mistake you're making is assuming "historically" applies to the US. We had one Civil War, where part of the existing government decided to fight other parts of the government. They then sent the citizens to fight (often against their own interest) against each other via conscription.
An actual war against the people of the United States by the United States by the standing military would be so utterly one sided as to be incomprehensible. Let me just put it this way: no one at Waco was stopping those tanks.
The mistake here is thinking of the U.S. government as a monolith. Ultimately it's all just people, bound together by being paid for in dollars that are either raised as taxes or borrowed as treasuries. GP's post posits a world where the dollar is worthless; what's binding them together then?