Abstract: May a threatened state use force against armed nonstate actors situated in another state without the other state's permission? Proponents of the “Unable or Unwilling Doctrine” ("UUD") answer in the affirmative, provided that the territorial state in which the nonstate actors are based is either unable or unwilling to tackle the threat by itself. Opponents reject the UUD, arguing that it has no place within existing international law. The intense, multi-layered debates over the UUD have thus far been grounded primarily in the international law of self-defense. Moreover, both proponents and opponents of the doctrine have tended to treat its two prongs as interchangeable, such that the legality of a use of force or the consequences that follow from it are unaffected by which of the two explains the territorial state’s failure to negate the threat to the targeted state. This Article challenges both of these features of UUD analysis. Our first contention is that, while states enjoy limited leeway to use defensive force against nonstate actors in another state’s territory, the prerogative to enter the territorial state without other authorization is rooted in principles of necessity, not self-defense. In turn—and here we reach our second main contention—grounding the UUD in necessity suggests that, for cases in which the territorial state is unable, rather than unwilling, to deal with the threat, the threatened state is obligated to compensate the territorial state for harm caused by its unpermitted entry. Our third contribution is to explain why compensation might be owed, as a matter of equity, even for the entry itself as a (justified) violation of sovereignty. All of these claims, we contend, are bolstered by interpreting international law through the lens of private law, particularly the Anglo-American law of tort and restitution and its rules for the imposition of liability in cases of "private necessity."