This question falsely presumes that the government's military is not also fractured by whatever is causing the militia to revolt. The military is made up of citizens.

But then why do you need the militia?

To protect yourself from and fight against weakened fractions of the government.

The military cannot occupy the entire space of the US so maintaining order of a tyrannical government would require a low ratio of military guards to citizens. With even 50% of the citizens potentially armed and willing to fight back, it's an untenable position for the government.

A militia could fight just as effectively as the Taliban against the government and be several orders of magnitude larger. The US military is good at eliminating identified enemies, but it's pretty terrible at nation building and that's even in its strongest state.

Because I want to a reason to own lots of guns.

Afghan militia has fared well both against US/NATO and Soviet militaries in the past 50 years.

If you are referring to the Taliban and their precursors, they have had, and continue to have significant financial and logistical backing from nation states.

Does that make them a bad example of a successful militia?

Perhaps a successful militia requires significant financial and logistical backing from nation states, in addition to being well trained.

In that case they are paramilitaries, not just militias.