Here’s the key point in a profile of Gene Sharp, who has written extensively on how to resist tyrants:
Peaceful protest is best, he says — not for any moral reason, but because violence provokes autocrats to crack down. “If you fight with violence,” Mr. Sharp said, “you are fighting with your enemy’s best weapon, and you may be a brave but dead hero.”
This is precisely why non-violent protest can work against some regimes but not others. Those regimes whose armed supporters are willing to kill peaceful protesters survive this kind of resistance just fine; those without the nerve to kill people wholesale end up retiring to Saudi Arabia, if they’re lucky. That’s why the ChiComs and the mullahs are still in charge, and why we’re not seeing a lot of “people power” in Syria. Now, a regime can continue for a long time after it has lost the will to kill, simply out of inertia — it takes a serious and sustained non-violent challenge to force it to confront the choice between mass slaughter and surrender. But once you force the question, as the Egyptian protesters did (and as the peoples of the former Soviet empire did 1988-1991), they’re finished.
This is why Gandhi and Martin Luther King were successful — not because of the power of pacifism as an ideology but because they were dealing with opposition that was simply too civilized to kill on the scale necessary to defeat them, even though such killing would have been quite easy to accomplish. And this is why preaching non-violent resistance to Hitler or Stalin wasn’t just silly, it was immoral; Niebuhr once wrote mockingly of someone calling for the use of “soul force” against the Nazis, asking how soul force was supposed to stop Stukas and Panzers. (If anyone knows the citation for that article, please send it to me; I’ve searched in vain for it.)
Soft-headed liberals (sorry for the redundancy) too often confuse non-violent protest with pacifism. In many cases, peaceful protest is an important tool in fighting for liberty. But renouncing the use of violence in principle, under any circumstances, simply makes one an accomplice to evil and is an abomination.