Like many absolute statements, this claim is just plain wrong. America (and many other countries) were started by revolutions. The revolutionaries had guns.

Countries like India are unique for gaining freedom without violence.

I mean just look at Syria. I don't have any feelings good or bad towards the rebels. But people have been trying to get rid of Assad for ages and it just took the right people with guns.

One man's violence is another's righteous revolution. All political power is at the end of the barrel of a gun.