This dissertation explains military effectiveness in executing population centric counterinsurgency (PCC) strategies. Why do some states conduct PCC effectively, while others attempt it, but fail to do so? Why are some states able to increase their effectiveness over the course of a campaign. In short, what are the determinates of PCC effectiveness? The dominant literature focuses on the effects of past formative wars on a military organization’s culture, the role of senior commanders, or the size of the military force and its ratio to the insurgent or civilian population. However, formative war based theories cannot explain within case variation, while commander and size based explanations are underspecified and underperform in empirical tests. Instead, this dissertation argues that PCC strategies create a principal-agent problem within the military organization. PCC requires subordinate units and soldiers to undertake dangerous activity, endure significant hardship, and forgo opportunities for personal gain, all in an environment of limited oversight. To prevent shirking and subverting of the strategy, militaries undertaking PCC require a command structure mirroring a Weberain bureaucracy, to include high levels of expertise, clear and well respected lines of hierarchical authority, and impersonal organizational and institutional loyalty. Expertise and hierarchy are necessary for monitoring, while impersonal loyalty is necessary to create a values and standards infused organizational culture that fosters agents self-motivated to achieve organizational ends. Merit based promotion and appointment systems are necessary to achieve this Weberian command structure. By contrast, promotion and appointment systems based on nepotism, cronyism, or patronage undermine these Weberian characteristics by ignoring expertise in leader selection, subverting formal chains of command, preventing monitoring and evaluation, and creating an organizational culture that privileges personal gain and loyalty to patrons over loyalty to the organizational mission. This dissertation probes the plausibility of the theory through two detailed case studies: the Huk Rebellion (1946-1956) and the Salvadoran Civil War (1979-1992). In the Philippines case, it uses longitudinal variation in the independent variable of promotion and appointment systems over the course of the war to demonstrate that changes in these systems cause a change in PCC effectiveness. In the Salvadoran case, it uses variation in the variables proposed in the dominant competing theories to show that by not accounting for promotion and appointment systems, those theories fall short in explaining PCC effectiveness.