Does reputation matter in world politics? If an American president draws a “redline” but fails to enforce it with military power when an adversary crosses it, will this embolden other adversaries? The prevailing wisdom among policymakers and pundits is that credibility counts. No such consensus exists, however, among political scientists, many of whom argue that Washington’s credibility in the eyes of an adversary will be overwhelmingly shaped by the adversary’s reading of current U.S. interests and capabilities, and not by past actions. Harvey and Mitton join this debate with a detailed study of the Obama administration’s response to Syria’s use of chemical weapons in 2013. After Syria crossed U.S. President Barack Obama’s publicly stated redline by using such weapons against civilians and rebels, Russia brokered an agreement in which Syria gave up its chemical stockpiles but avoided U.S. military action. Harvey and Mitton argue that Washington’s “reputation for resolve”—earned by the use of U.S. military force in similar circumstances in Bosnia, Kosovo, and Iraq—influenced Russian and Syrian calculations. The authors do not settle the debate about whether Obama harmed that reputation by declining to use military force to punish Syria. But they drive home the fact that credibility ultimately depends on the imperfect perceptions of leaders and their real-time calculations of risks and probabilities.