We have modern examples of this situation, and that’s not how it works. In the 1971 Bangladesh independence war, the Pakistani military started conducting a genocide of Bangladeshis. (They didn’t do it with fighter jets, which they had plenty of, but infantry with rifles.) Bangladesh was mostly disarmed, but arms were funneled to it from India. Civilians also raised army supply depots to steal guns. They then put up a resistance with the arms they had or could get. That caused large swaths of the Pakistani military—many of whom were Bangladeshis—to defect and fight for independence. A citizen military’s willingness to fight their own people is limited. Having enough arms to start the armed resistance was the most important thing. And I suspect a lot fewer Bangladeshis would have been killed in the genocide had they been as well-armed as Americans are.

At the end of the day, holding territory is hard. Look at how much trouble Americans have had subduing the Taliban in Afghanistan. The bombs, tanks, planes, etc., are largely useless in holding a city. Now, combine that with the fact that the US military is comprised of citizen soldiers, who would be in the position of fighting against their own communities.

It’s when a community is totally disarmed, and when the mere show of military force would force total capitulation without violence, that a citizen military could rationalize aiding tyranny.

It’s also worth pointing out that there are 50 governors who command national guards that have fighter planes and tanks and whatnot. In a real civil war, armed citizenry would likely be fighting alongside state forces.