Attempts have been made to develop some general understanding, and ultimately a theory, of systems that consist of many interacting components and many hierarchical layers. It is common to call these systems complex because it is impossible to reduce the overall behaviour of the system to a set of properties characterising the individual components. Interaction is able to produce properties at the collective level that are simply not present when the components are considered individually. Warfare is a complex system that is linked and interacts (in a coevolving way) with the surrounding socioeconomical and political context. Forgetting that war and warfare are an intimate part of a much larger complex system will lead to incomplete and even dangerously incorrect conclusions. Applying the approach of Complexity Theory to warfare leads one to the self-consistent realisation that warfare will have to be analysed in its larger context. Further work will need to examine how coevolution across the entire network of military, socioeconomical, and political interactions leads firstly to emergent effects at higher levels, and of equal importance how such effects lead to coevolution at the higher level. It will also be important to consider the robustness of such networks, and their vulnerability to damage.