The creation of an International Criminal Court (ICC) to prosecute war crimes poses a real puzzle. Why was it created, and more importantly, why do states agree to join this institution? The ICC represents a serious intrusion into a traditional arena of state sovereignty: the right to administer justice to one’s one nationals. Yet more than one hundred states have joined. Social scientists are hardly of one mind about this institution, arguing that it is (alternately) dangerous or irrelevant to achieving its main purposes: justice, peace, and stability. By contrast, we theorize the ICC as a mechanism to assist states in self-binding, and draw on credible commitments theory to understand who commits to the ICC, and the early consequences of such commitments. This approach explains a counterintuitive finding: the states that are both the least and the most vulnerable to the possibility of an ICC case affecting their citizens have committed most readily to the ICC, while potentially vulnerable states with credible alternative means to hold leaders accountable do not. Similarly, ratification of the ICC is associated with tentative steps toward violence reduction and peace in those countries precisely least likely to be able to commit credibly to forswear atrocities. These findings support the potential usefulness of the ICC as a mechanism for some governments to commit to ratchet down violence and get on the road to peaceful negotiations.