Actually, I have a little experience on that. The main thing is that it's pretty hard to convince "modern military" to kill their own families or neighbors. So, in case like Belarus, if you order your military to fight like 90% of population, you have very high risk of them turning the arms the other way.

But if those 90% of population can be silenced by a very small filtered force (personal guards, but better call them "riot police"), and keep the real "military" in barracks fed by propaganda 24/7, preventing talking to people -- then they would learn the truth anyway, but when it's too late, when all the leaders are already dead/jailed/exiled.

The whole point of legitimacy rests on public approval. What approval are we talking about if it needs "A-10 Thunderbolt" against your own people. If an american city, say, Portland, would start armed resistance, no President would ever order airstrikes against it, that would be the last day of their presidency. But they won't start it, because the whole system already accommodates Portland wishes into non-violent resolution.

A side note: Napoleon military genius rests a lot on the La Grande Armée, the largest army by far in Europe. But why other countries didn't have their own armies of the same size? Because only democracy can keep such a big army to turn the arms against its government, otherwise at some point giant army wishes may not align with its government (although it may take time of course). Democracy at least persuades people that they have a voice. So, in some sense, the Napoleonic wars made "democracy -> large army size -> democratic neighbors" transition.

Going back to Belarus example, if the "riot police" would have to deal with armed protestors, they would have it like million times harder, and I'm not sure of the outcome.