Uncertainty about military power is widely considered an important determinant of international conflict, but research in international relations provides relatively little guidance about the origins of uncertainty. What factors influence the validity of actors’ assessments of military capabilities? When would one expect uncertainty about military capabilities to be particularly high, or especially low? We examine a series of factors capable of explaining the sources of uncertainty in international relations, positing that the uncertainty of assessments is a function of both characteristics of the state being assessed and the overall strategic environment. We test our theory using new measures of uncertainty that we derive from analyzing publicly available estimates of national military capabilities. Our findings offer a novel set of implications both for research that relies on uncertainty as a key cause of conflict and for policymakers faced with evaluating foreign military assessments.

There is a growing sense that US military effectiveness has been on the wane in recent years. Is this the case? If so, what are the reasons for the decay in American combat performance? We first examine the available systematic evidence for American military decline, showing that the United States has indeed experienced a drop in the quality of outcomes of its military contests. Observers have offered a number of explanations for declining American military success, most predominantly an increase in intrastate conflict after the Second World War. After showing that a decline in performance is observed even after fully excluding intrastate conflict, we propose an alternative explanation: the increasing distance from home at which the United States has been fighting. Distance is tyrannical: it saps military strength and increases the cost of contests, even as it reduces US expertise and motivations to prevail. We then show that the distance from home at which the United States fights is the best predictor of the outcome of the conflict. We conclude by noting some avenues for future research and policy implications as the world returns to great power competition.