Why do militaries invest in some emerging technologies but not others? Conventional wisdom suggests that capable states have reasons to hedge their bets and invest in emerging military technologies as widely as they can. Yet, even the most capable states do not invest in all technologies of military utility. Moreover, in some cases, early investments in research and development (R&D) are not sufficiently sustained to lead to any realized capability. This dissertation answers the question of why some emerging technologies are able to attract and sustain military investment while others cannot. I argue that decisions over such investments are influenced by relevant actors’ assessment of feasibility during the R&D process, the military requirement the technology fulfills, and the availability of alternatives. In particular, a dominant belief in low feasibility, highly stringent requirements, and available alternatives can create an unfavorable condition that makes an emerging technology unappealing as an investment opportunity. Such a condition can prevent a technology from attracting or sustaining investment even if it were to have legitimate military use. Three case studies are conducted to illustrate this argument: biochemical non-lethal weapons, neuropharmacological treatment for combat stress, and aircraft nuclear propulsion. This dissertation yields important policy implications for understanding state investment behavior and managing defense R&D in emerging military technologies.