The connection between political philosophy and war has always been, and continues to be, intricate and multifaceted and frequently paradoxical. For a long time, philosophers have wrestled with the causes, ethics, significance and consequences of war, and their views span a wide spectrum—from justifying it under certain conditions to condemning it outright. This chapter aims to map this bi-millennial tradition in its essentials, on the basis of the fundamental question of whether war can be justified. Needless to say, this is just one possible starting point for exploring the problem of armed conflict, which is in turn one of the most profound and contested questions in ethics, politics and history. Furthermore, the answer (or the set of answers) significantly depends on the adopted perspective—practical, moral or legal. That said, taking a generally normative standpoint and limiting our analysis to outlining the key distinctive features of each of them, we can identify five possible answers (i.e. five fundamental theories) to the question raised above: just war theory, pacifism, political realism, bellicism and what can be labelled as holy war theory. As with any mapping exercise, the aim is to produce the most precise and comprehensive cartography possible (which, of course, is always an exercise in abstraction). However, exploring the normative justifiability and argumentative cogency of the approaches we are going to consider, as well as their historical roots and development over time, is beyond the scope of this contribution. The only relevant question to which a comprehensive answer will be attempted is the following: How many philosophical approaches exist to the issue of the legitimacy of war?