Vietnam is a bad example...
The greatest military's in the last century did in fact oppress the populations of their corresponding countries.. There were only 2 comparable military forces, and both succeeded.... USSR, Nazi Germany. Both succeeded.
In both cases the military had a key role on getting the tyrants into power in the first place. And in both cases the population did have resistance, but on very small scale, since repression against families of the resistance members was carried out with brutal force.
You would keep fighting if they'd take your children, your grand parents, and execute them for no reason other than having you stop fighting?
Some will...... Most will surrender. it's better to live within a regime, than to die or have those you love die for you.