Replication Data for: The Tyranny of Distance: Assessing and Explaining the Apparent Decline in U.S. Military Performance

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.近年来，有关美国军事效能日趋式微的论调日益高涨。这一论断是否属实？若属实，美军作战表现下滑的根源又是什么？ 本文首先梳理了可获取的系统性实证证据以验证美军战力下滑的论断，结果显示美国确实在其参与的军事行动中出现了作战成果质量的下滑。针对美军作战胜率下滑的现象，学界与观察人士已提出诸多解释，其中最具代表性的是二战后国内冲突（intrastate conflict）发生率的上升。在验证了即便完全剔除国内冲突样本，美军作战表现依然存在下滑趋势后，本文提出了另一解释：美军的作战地域与本土的距离不断增大。作战距离堪称严苛桎梏：它不仅会侵蚀军力、抬升作战成本，还会削弱美军的作战专业素养与取胜意愿。随后本文验证发现，美军作战地域与本土的距离，是预测冲突作战结果的最优指标。最后，在全球重回大国竞争格局的当下，本文总结了若干未来研究方向与相关政策启示。