Just-Warists and War-Pacifists disagree on whether soldiers are morally justified in killing each other in wartime combat. Many of their respective arguments, and their contradictory conclusions, are based upon principles of self-defense. In this thesis, I examine the role that principles of self-defense play in the arguments surrounding the moral justification of killing in combat. I do so by critiquing both a Just-Warist argument that relies on self-defense (constructed from the works of Michael Walzer and Judith Jarvis Thomson) and a War-Pacifist argument (developed by Richard Norman) that condemns killing in combat based on the moral requirements of self-defense. I demonstrate that both arguments fail due to their mistaken assumptions that soldiers are not morally responsible for their actions. I conclude by arguing that--once soldiers are recognized as morally responsible agents--killing in combat can be morally justified by principles of self-defense.