The reconnaissance strike complex is synonymous with modern military power, and prominent realist theories would have predicted rapid proliferation after its successful debut in the Gulf War. Instead, the complex has proliferated slowly. To explain this puzzle, we theorize that interstate security threats significantly impact proliferation, but not in the way traditionally presumed. Although the literature on weapons proliferation has largely assumed a monotonically increasing relationship should hold between the capabilities of a state’s adversaries and a state’s own capability, we draw from the economics literature and game theoretic insights from political science to argue that the relationship should resemble an inverted-U. When states have rivals with moderate reconnaissance strike capabilities, they have security incentives to compete with them. However, when states face highly advanced adversaries, it becomes more difficult to escape or match their competition, making symmetrical acquisition less appealing. While most prior research focuses on narrower aspects of the reconnaissance strike complex like missiles or smart bombs, we test our theory on a novel dataset tracking country-level acquisition of eight aspects of the complex from 1980 to 2017: Ballistic missiles; bombers; cruise missiles; fighter aircraft; intelligence, surveillance, and reconnaissance assets; precision-guided munitions; satellites; and submarines. We find strong support for our inverted-U argument. States that have rivals with moderate reconnaissance strike capabilities have over double the reconnaissance strike capabilities themselves than states that have rivals with very low or very high capabilities. Our findings hold for broader measures of the complex that closely proxy a state’s general military capabilities, narrower measures of the complex, and alternative measures of general military sophistication, indicating our theory has broad applicability. This article explains why some states invest heavily in conventional capabilities despite an already-large lead over their adversaries, and why other states instead opt to invest in alternatives rather than balancing symmetrically.