BUT...In countries that have fallen to dictatorial tyranny, the enemy isn't a foreign invader. One of the major tools a dictator uses to spread fear and to control the population is informers. I doubt that in this situation, a resistance group could grow large enough to have any effect without first being infiltrated by the dictator's informers.
The French resistance was made up of many disparate elements: Democratic types, socialists, communists, Royalists, anarchist, nationalists and many others. Yes, many of their number were fingered and turned over to the Germans, but many more were not.
That’s why the groups would have to remain small and autonomous, like the Tea Party.
It drives the left nuts that they can’t find “the leader” of the Tea Party and single him out for destruction.