Abstract: This study examines processes of military doctrinal change during periods of peace. Given the conventional wisdom of hidebound bureaucratic military organizations, why do these organizations innovate doctrinally? Rather than conduct competitive hypothesis testing between two or more theories of military innovation or pursue a heretofore undiscovered monocausal theory, I develop and test a theoretical framework that synthesizes more than one approach to military doctrinal innovation. I use this framework to conduct a structured, focused, case-study comparison of two military organizations - the U.S. Army and the U.S. Marine Corps - from the post-World War II period until 2001. The study yields seven findings. First, the systemic causes of military doctrinal innovation are best described by balance of threat theory. Second, contrary to the existing literature, civilian intervention is not a necessary or sufficient cause of doctrinal innovation. Third, militaries consistently strive to establish a monopoly over warfare in a particular jurisdictional domain. Fourth, the frequency of military doctrinal change is a function of the complexity of the strategic problem that doctrine is designed to solve. Fifth, the complexity of the cases studied supports the argument that monocausal explanations fail to account for the interaction of multiple variables that affect doctrinal innovation. Sixth, military doctrinal innovation during peacetime is not anomalous because military organizations constantly revise their theories of victory as threats change in the external environment. Finally, the existence of doctrinal institutions creates a norm for a reliance on military doctrine.