text: General Background and Basic Principles of the Law of Armed Conflict
  1-21. LOAC principles work as interdependent and reinforcing parts of a coherent system. Military necessity
  justifies certain actions necessary to defeat the enemy as quickly and efficiently as possible that are not
  prohibited by LOAC. Conversely, humanity forbids certain actions unnecessary to achieve that object.
  Distinction underpins the parties’ responsibility to comport their behavior with military necessity, humanity,
  and proportionality by requiring parties to a conflict to recognize and respect certain legal categories:
  principally, the distinctions between combatants and civilians, and between military objectives and civilian
  objects. Proportionality requires that even when actions may be justified by military necessity, such actions
  cannot result in excessive civilian loss. Lastly, honor supports the entire system and gives parties confidence
  in it, and provides legitimacy to the entire endeavor.
  1-22. LOAC principles are consistent with military doctrine that are the basis for effective combat
  operations, and the ethical standards of the military profession. Commanders can use LOAC principles to
  guide them in making difficult decisions and judgments in military operations. For example, a commander
  might consider whether there is a legitimate military purpose for an action; the proposed course of action is
  unreasonable or excessive; or, are there precautions that can be taken to avoid unnecessary suffering.
  MILITARY NECESSITY
  Military Necessity
  Military necessity permits a belligerent, subject to the laws of war, to apply any amount
  and kind of force to compel the complete submission of the enemy with the least possible
  expenditure of time, life, and money.
  List Case, World War II
  1-23. Military necessity is the principle that justifies the use of all measures needed to defeat the enemy
  as quickly and efficiently as possible that are not prohibited by LOAC (see DOD Law of War Manual,
  2.2; see Lieber Code, articles 15 and 16 for historical reference).
  Military Necessity as Justification
  1-24. Military necessity justifies actions, such as seizing persons and destroying property. Thus, military
  necessity underlies LOAC concepts that explain when persons and property may be the object of attack, such
  as the concepts of “direct participation in hostilities” (see paragraphs 2-11 to 2-21) and “military objective”
  (see paragraphs 2-29 to 2-57). Military necessity may justify not only violence and destruction, but also
  alternative means of subduing the enemy. Military necessity also permits certain incidental harms that
  inevitably result from the actions it justifies. Military necessity may justify the use of overwhelming force to
  defeat enemy forces because the object of war is not simply to prevail, but to prevail as quickly and efficiently
  as possible. Military necessity does not require commanders to use the minimum force necessary in a given
  situation; such an interpretation of military necessity would prolong the fighting and increase suffering.
  Military Necessity and the Law of Armed Conflict Rules
  1-25. Military necessity does not justify actions that are prohibited by LOAC. In particular, military necessity
  cannot justify departures from LOAC because States have crafted LOAC specifically with war’s exigencies
  in mind. Prohibitions in LOAC may be understood to reflect States’ determinations that those prohibited acts
  are always militarily unnecessary or otherwise inappropriate or unlawful.
  1-26. Although military necessity cannot justify actions that have been prohibited by LOAC (see, for
  example, GC art. 49 (prohibiting forcible population transfers from occupied territory)), some LOAC rules
  expressly incorporate military necessity. For example, certain LOAC rules specify that departures from what
  would otherwise be the rule are permissible when absolutely or imperatively necessary (see, for example,
  GWS art. 8 (restricting activities of protecting power representatives); GC art. 42 (interning protected
  persons), and art. 53 (destruction of property during occupation); HR art. 23(g) (seizing enemy property)). In
  such cases, military necessity must not be conflated with military convenience. As another example, certain
  LOAC rules may direct that persons comply with an obligation, but only to the extent feasible or consistent
  with military necessity (see, for example, GWS art. 30 (repatriating retained personnel); GC art. 108 (limiting
  relief shipments)).