Virtually all interstate and civil wars involve significant periods during which the combatants simply fight and do not exchange serious offers.  In a model where a government cannot commit against changing its proposal if a rebel group accepts, a "ratchet effect" is shown to undermine the government's ability to screen militarily weak adversaries via offers, leading it to use fighting to screen weak types.  The argument provides an explanation for important facts about armed conflict that are puzzling for standard incomplete-information bargaining models (which do not explain long war duration and non-serious offers) and commitment-problem models (which do not explain eventual negotiated settlements due to battlefield learning).