Exploring the profound differences between what the military services believe—and how they uniquely serve the nation. When the US military confronts pressing security challenges, the Army, Navy, Air Force, and Marine Corps often react differently as they advise and execute civilian defense policies. Conventional wisdom holds that these dynamics tend to reflect a competition for prestige, influence, and dollars. Such interservice rivalries, however, are only a fraction of the real story. In Four Guardians, Jeffrey W. Donnithorne argues that the services act instead as principled agents, interpreting policies in ways that reflect their unique cultures and patterns of belief. Chapter-length portraits of each service highlight the influence of operational environment ("nature") and political history ("nurture") in shaping each service's cultural worldview. The book also offers two important case studies of civil-military policymaking: one, the little-known story of the creation of the Rapid Deployment Joint Task Force in the early 1980s; the other, the four-year political battle that led to the passage of the Goldwater-Nichols Act in 1986. Donnithorne uses these cases to demonstrate the principled agent framework in action while amply revealing the four services as distinctly different political actors. Combining crisp insight and empirical depth with engaging military history, Four Guardians provides practical utility for civil-military scholars, national security practitioners, and interested citizens alike. This timely work brings a new appreciation for the American military, the complex dynamics of civilian control, and the principled ways in which the four guardian services defend their nation.