I would think that anyone serious about secession would contemplate that it would be difficult and perhaps result in a fight, of some kind....
Well, all the historical examples I can conjure did, so seceders should certainly be prepared for it, but I can also see situations where both of the parties felt separation to be in their respective interests, so I don't think a peaceful separation is impossible. (Not talking about this or any tangible case, just the concept of secession/political separation in general)