The Civil War certainly settled the question for the time being; though it is not a question that can ever be permanently settled.
Force is certainly one factor that answers the question. Force helped answer the implicit question posed in the Declaration of Independence: 'when in the course of human events it becomes necessary....' The colonists that penned and lived by those words believed that time had come; England obviously felt otherwise. The contest between the 2 views was settled largely by force.
When does it become necessary to separate? That depends. So long as there is no world court able to determine when an allegedly consensual contract is permitted to be revoked by one of the parties, war will mostly decide.