Wars begin when diplomacy fails. And in most cases of interstate conflict, wars end after diplomacy resumes. Research on war has made tremendous progress in understanding why states abandon the bargaining table and resort to fighting even when a peaceful and less costly deal theoretically exists. Much less has been said about when and why belligerents choose to return to that table. We consequently have limited capacity to understand conflict resolution or to offer practical counsel to policymakers.

In The Costs of Conversation, Oriana Skylar Mastro takes a significant step forward in addressing these issues and in challenging a common yet empirically dubious view in international relations scholarship that wartime diplomacy is “cheap talk.” Mastro outlines what she calls the Costly Conversations Thesis (CCT), which explains why belligerents adopt an open or closed diplomatic posture during war—that is, whether or not they are willing to engage in direct and unconditional negotiations. The key claim of the CCT is that states may refuse to negotiate while fighting because they fear that it is costly in signaling one's weakness and a reduction in one's war aims.

Two factors guide this calculation. The first is the risk of adverse in