Thesis advisor: Shep Melnick / Most of the literature on the separation of powers in the war on terror vastly overstates the power of the presidency and pays little attention to the respective roles of Congress, the courts, and the bureaucracy in prosecuting that conflict. Scholars – especially those in the legal academy – have consistently failed to appreciate the ways in which the president has been, and continues to be, checked and constrained by a variety of forces. In my dissertation, I engage in highly detailed case studies of U.S. law and policy with respect to detention and military commissions in the war on terror. I pay special attention to the complex interactions that occurred within and between our governing institutions in these policy areas. There are two central arguments that come out of my research and run through my case studies. First, the political scientist Robert Kagan’s work on “adversarial legalism” is no longer simply applicable to the domestic policy realm. The proliferation of legal rules and extensive litigation has increasingly come to characterize foreign affairs as well, with important consequences for how the U.S. implements its national security policies and fights its armed conflicts. In short, adversarial legalism has gone to war. Second, loose talk about the “unitary” nature of the executive branch is misleading. The executive branch is a sprawling bureaucracy made up of diverse actors with different perspectives, preferences, and norms, and that bureaucracy has interacted with Congress and the courts in surprising ways to constrain the presidency in the war on terror. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Political Science.