The concept of necessity is frequently invoked in the context of war and conflict and in the context of the application of force more broadly. References by politicians, lawyers and military officers to a necessity that compels (non-)action are numerous in circumstances of violent crisis situations such as wars and conflicts. Despite this backdrop, the concept of necessity has, with few exceptions, largely escaped systematic study. The purpose of this thesis is to examine how necessity is understood in the context of war and conflict and investigates the extant concept across five separate knowledge communities - ad bellum & in bello international law, military strategy, Realpolitik and the Just War tradition. Using methodological tools of Begriffsgeschichte this thesis treats necessity as a contested concept and examines how necessity is understood in each respective discourse and elucidate whether necessity has a uniform meaning or is a function of the discourse it is invoked in. This thesis will show that necessity is differently understood in each respective knowledge community and invoked to differing effects and for different purposes, thereby providing a more nuanced explanation of ways in which the (non-)use of force is justified and legitimised.