If it's created out of land that they have taken within that same war, I wouldn't call it a strategic secession. Creating puppet states (marches would be a medieval appropriate term) in the wake of your army's passage is a legitimate war tactic.
If it's created out of lands that were held by the parent realm in peace time, then it could be a strategic secession.
Well, part of the point here is the "too far away from their own capital not to revolt almost immediately", I think...
