Russia and Ukraine

[BrianSoddingBoru4]

This ignores the fact that aside from the self-employed (a small minority of tax-payers), Federal taxes are collected by payroll deductions, meaning that unless said taxpayers quit their jobs, the Feds receive a constant influx of money, and military paychecks roll out on schedule.

Taxpayers usually don't work during revolutions.

Unless you're destroying powerlines in DC, you're not eliminating the disbursement of military salaries, which are by law nowadays by direct deposit. As for the local ability of soldiers to get money from local ATMs, they don't need to in the field, as they're fed and supplied by the logistics of the military. They are also often paid a per-diem, in cash, for daily needs not supplied by the logistics train.

It would be one of the main targets

So you'll have to strike at the financial system itself in order to hamstring a military response. And that has pretty robust defenses built-in.

What defences? Financial sector can't work if there is no electricity.

Apples and oranges. Your two examples feature nations of very different languages, ethnicities, and financial systems.

US military couldn't defeat backward countries despite full support of its huge economy. Rebellion would leave US economy in ruins.

You're also ignoring the fact that most Americans would not see the American military as an occupying force to be fought, but rather the legitimate arm of governmental force.

It's unlikely that the majority of Americans would support a tyrant.

(Bold mine)

Then why do they keep getting elected?

Boru