This thesis examines the complexity of civil-military relations through an inno- vative lens of the Principal-Agent theory that considers two agents: the military and the civilian workforce within the Ministries of Defense. The study presents an in-depth compar- ative analysis of Brazil, France, and the United States, focusing on the dynamics between these two agents. Our findings delineate stark variations between the case studies. In Brazil, the Ministry of Defense operates primarily under a single-agent model, with the military being the dominant force, thereby leading to potential policy biases. In contrast, France and the United States present a more balanced power distribution between the military and civilian workforce within their Ministries of Defense. This balanced model promotes healthier civil-military relations, more diverse perspectives, and effective policy-making. The strong presence of civilians as a second agent in France and the US represents an evolution in traditional Principal-Agent frameworks in civil-military relations.