Violence is justifiable in some extreme cases in order to combat violence.  In other words, if someone is being cruel and destructive, it is acceptable to me, morally, to use violence to protect people.  It is kind of like shooting in self-defense.

I think that it is not typically justifiable to start a war.  However, there are times when it could be.  I think that violence and war would be justifiable, for example, as a way of stopping some horrible evil like the Holocaust.  War is also justifiable when one's country is attacked and endangered.