The duty to capture stands at the fault line between competing legal regimes that might govern targeted killings. If human-rights law and domestic law-enforcement procedures govern these killings, the duty to attempt capture prior to lethal force represents a cardinal rule that is systematically violated by these operations. On the other hand, if the law of war applies then the duty to capture is fundamentally inconsistent with the summary killing already sanctioned by jus in bello. The following Article examines the duty to capture and the divergent approaches that each legal regime takes to this normative requirement, and evaluates internal debates within these regimes over when a duty to capture might apply. At issue in these debates, regardless of the body of law that applies, is the scope and content of the concept of necessity: when it is truly necessary to target an individual with lethal force. The key question is whether a unified and trans-regime understanding of the concept could promote doctrinal unity across legal regimes. However, this Article concludes that the concept of necessity stubbornly defies such attempts; necessity is a term of art with a distinct history and meaning in each body of law, and unification of these meanings can only come at the cost of betraying the fundamental precepts of one legal regime over the other. Part I begins by examining the scope of international humanitarian law (IHL); concludes that its application is often unduly constrained; and offers a new analysis of the classification of armed conflicts, the level of organization required before a non-state actor can be a party to an armed conflict, and the legal geography of armed conflict. Part II examines the concept of necessity and concludes that military necessity (destruction of “life and limb” related to the war aim) is fundamentally incompatible with human-rights law and its understanding of necessity as the least-restrictive means. Finally, Part III concludes that the IHL regime, and its permissive notion of military necessity, should apply when the state is acting as a belligerent against other co-equal belligerents, but that human-rights law, and its more restrictive notion of necessity, should apply when the state acts as a sovereign over its own subjects. However, being a U.S. citizen does not automatically make an individual a “subject” under a sovereign, as opposed to a belligerent. Rather, this Article concludes that belligerency is always a relationship between collectives, and that the relevant question is whether the United States stands in a relationship of belligerency to a non-state organization of which the individual is a member.