Revolution

I think that revolution can come in many forms, political, social, and so on.  However, I do think that violence tends to go along with any kind of drastic change.  Sometimes violence comes on the part of the revolutionaries, when they believe they must wage war against the established system in order to achieve their goals.  Other times the violence comes from the resisting side.  Peaceful protesters asking for radical change will often incite violence from the establishment who wants to stop them from speaking.
I would also say that revolutions are constantly happening.  As soon as one revolution succeeds and becomes the establishment another will begin with the intent of overthrowing it.  Of course it isn't always easy to see the revolutions, and the same type may not always follow the one before it. (For example: A revolution such as the Civil War (successful or not, I would still consider this a revolution) was followed by a social revolution like the Civil Rights Movement.)  But I do think that nearly every revolution has had violence at some point.  With the exception, perhaps, of the Digital Revolution, if one considers that a  real revolution.