As Conor Cruise O’Brien once remarked, “Violence is sometimes needed for the voice of moderation to be heard.” And indeed, violence, contrary to what its prophets try to tell us, is a much more effective weapon of reformers than of revolutionists. (The often vehement denunciations of violence by Marxists did not spring from humane motives but from their awareness that revolutions are not the result of conspiracies and violent action.) France would not have received the most radical reform bill since Napoleon to change her antiquated education system without the riots of the French students [in May 1968], and no one would have dreamed of yielding to reforms of Columbia University without the riots during the [1968] spring term.

Still, the danger of the practice of violence, even if it moves consciously within a non-extremist framework of short-term goals, will always be that the means overwhelm the end. If goals are not achieved rapidly, the result will not merely be defeat but the introduction of the practice of violence into the whole body politic. Action is irreversible, and a return to the status quo in case of defeat is always unlikely. The practice of violence, like all action, changes the world, but the most probable change is a more violent world.

http://www.nybooks.com/articles/archives/2013/jul/11/hannah-arendt-reflections-violence/