Sociology can analyze war and warfare under different aspects, for example, as a problem of collective violence. It has rather neglected another important aspect of war, as in the fact that war is also always an organizational phenomenon. In the last few years, several studies on war have been focusing on this aspect by using or referring to Niklas Luhmann’s system theory. This paper looks at some of these aspects by critically asking how these sociological studies use Luhmann’s theory in their analysis of war or war-related social structures. Luhmann’s theory, particularly the theory of society based on the principle of functional differentiation, has a powerful explicative potential, particularly for the analysis of war and warfare. However, only a few studies are actually using Luhmann’s theory in an adequate way by situating the concepts at the correct analytical level. War and “military systems” should be analyzed as organizational structures in society which are managed, first of all, by the political system, a function system of society, and which by no means excludes a multiplicity of interdependen-cies with other systems. A systemic perspective should also take regional “expressions” of society such as “military systems” in specific states or groups of states into account.