Strategic competition creates significant evolutionary pressures on conventional militaries to improve their doctrines and overall military effectiveness. Why are some militaries more effective at evolving their doctrine than others? This paper develops a theory which argues that complex security environments force militaries to optimize their organizations for the commitment horizon they possess for a security issue. This optimization drives militaries to take on particular organizational characteristics, such as the amount of delegation and the tolerance for experimenting with new tactics, which affect their performance in systematic ways as a result. Using an agent-based model, this study evaluates the theory by simulating different organizational characteristics and evaluating how doctrinal effectiveness changes based on different commitment horizons. Results from the simulations suggest a typology of militaries according to their commitment horizon. Contrary to existing arguments that military effectiveness is determined by a particular set of strategies or by a favorable military culture, the results also suggest that doctrinal effectiveness follows a U-shaped curve as the commitment horizon increases. To illustrate the logic of the model, this study describes the typology and its implications.