Reputation has long been considered central to international relations, but unobservability, strategic selection, and endogeneity have handicapped quantitative research. A rare source of haphazard variation in the cultural origins of leaders—the fact that one-third of US presidents were raised in the American South, a well-studied example of a culture of honor—provides an opportunity to identify the effects of heightened concern for reputation for resolve. A formal theory that yields several testable predictions while accounting for unobserved selection into disputes is offered. The theory is illustrated through a comparison of presidents John F. Kennedy and Lyndon B. Johnson and systematically tested using matching, permutation inference, and the nonparametric combination of tests. Interstate conflicts under Southern presidents are shown to be twice as likely to involve uses of force, last on average twice as long, and are three times more likely to end in victory for the United States than disputes under non-Southern presidents. Other characteristics of Southern presidencies do not seem able to account for this pattern of results. The results provide evidence that concern for reputation is an important cause of interstate conflict behavior.