Protracted wars (i.e. long slow wars via hit-and-run and guerilla warfare) have been very successful in past to topal the mighty powers by far weaker forces. However, in recent times, I feel they are becoming much less effective. The electronic surveylence and automated targetting makes such warfare much difficult. This is especially the case because, (1) the ruling power will eventually control all communication and remove any free speech, (2) they will constantly find and kill leaders through electronic means which are now amazingly effective.

A much better defensive route is probably mandatory militery training of all able citizens. Eventually, most professional armies are in the size of million at most and only 100,000 or so get deployed. If you have a million citizens with militery training, you are way harder to beat.