The question at the center of this dissertation is: Why do militaries change or preserve their doctrine? A new theory, Military Realism, is proposed to answer this question. Military Realism argues that major doctrinal changes are instituted by senior military leaders when the theories of victory and mission priorities of existing doctrine do not plausibly address the most dangerous threats. The degree of danger is based on the balance of relative material capabilities. Military leaders will make these changes independent of civilian intervention in doctrine. When civilian demands for doctrinal change are consistent with those underway in the military they will accelerate them. However, civilian intervention will not determine the essential characteristics of major doctrinal changes. Senior military leaders will also overcome bureaucratic preferences and the traditions of their military culture to implement the required changes. For instance, these leaders will design defensive, rather than offensive, doctrines despite bureaucratic and cultural preferences for offense. This dissertation tests the process and outcome predictions of Military Realism against a number of alternative explanations though US Army doctrine from 1960 to 2008.