It wouldn't be 100% of the military that would likely fall to tyranny or be controlled by a tyrant - there would be splintering and sides taken, who then can support and organize civilian militias.

There's really too much nuance and variables to argue this easily such as state of society, how rich or impoverished they are, how healthy or unhealthy they are - including their education - how open their heart and mind is, and so on.

A strong stable or stabilizing system - democracy in this case - that is capable of returning to the homeostasis with a forward-evolving movement will take an impressive attack to knock it down.

Of course there are bad actors who are trying to do such a thing, and due to all factors Biden won the election - and the Democrats won the Senate as well - so they can implement policy without being blocked by Republicans (nor extorted by giving $ trillions to corporations leveraging the need for stimulus-survival cheques) - and hopefully they implement new policies that will strengthen America and its democracy.

I believe Andrew Yang's core policies are the answer to breaking apart the duopoly and taking away the amalgamated power of the small handful of mainstream media channels (TV and newspapers) that amplify and support the duopoly: Democracy Dollars, Journalism Dollars, Ranked Choice Voting, Freedom Dividend/UBI, and Government Healthcare Option ("Medicare For All") to compete against the status quo - so people can become healthy and do their best to prevent dis-ease.

I agree with you that the State is an additional layer to reduce, limit, de-risk the potential of tyranny - perhaps then private gun ownership then becomes a layer against each State potentially becoming tyrannical, and if the State starts clamping down on people with bad behaviour - control-oppression-violence - to too severe of a degree, then arguably people will start moving to another State that is in line with their own values.

It's all about organization, alignment, and readying for defending against the different layers of potential conflict - whether citizen to citizen, citizen to State, citizen to Federal, State to Federal, Federal to Federal - the fight for democracy-freedom vs. tyranny, globally.

Edit to add: It's important when reasoning to understand how these situations unfold - they don't happen overnight and there is time to organize. There will be peak moments that act as signals - whether things are evolving or devolving: Biden was elected is one signal from one side, the continued attempted coup the other day, where sadly a handful of people died, is another signal from the side; like the genocide of Uighurs in China by the CCP is a signal from one side, on global scale. Generally the side that acts first in a physical violence means will be the less rational one, more aggressive and using anger which will blind them to some degree - "blind rage." This is why economic pressure via multi-lateral trade agreements between Democracies is the best starting solution - and with other policy can quickly reenforce the internal strength of those currently leaking economic value, ROI et al, to a bad actor who won't fall in line with freedom, health, non-violence, and so on; none of this is to say the US isn't guilty in some areas of unnecessary violence.