text: General Background and Basic Principles of the Law of Armed Conflict
  PROPORTIONALITY
  1-44. Proportionality is the LOAC principle requiring combatants to refrain from attacks in which the
  expected loss or injury to civilians and damage to civilian objects incidental to such attacks would be
  excessive in relation to the concrete and direct military advantage expected to be gained. It also
  underlies the requirement to take feasible precautions to reduce the risk of harm to civilians, other protected
  persons and civilian objects. In other words, even when a party to the conflict is justified in targeting a
  particular military objective, that party must not cause incidental harm to civilians or to civilian objects
  excessive in relation to the concrete and direct military advantage expected to be gained from targeting the
  military objective. The principle of proportionality complements the principle of distinction and, like
  distinction, derives from and applies both military necessity and humanity.
  1-45. Proportionality most often refers to the jus in bello (the law related to the conduct of hostilities)
  requirement to avoid excessive harm to civilians incidental to attacks, often referred to as “collateral
  damage.” It is this jus in bello proportionality principle, which underlies the requirement to take feasible
  precautions, that is discussed here and addressed in this publication. It is a different standard from the
  proportionality principle in the jus ad bellum (the law related to the use of force) principle of proportionality
  (see para. 1-2), which is beyond the scope of this publication.
  1-46. In war, incidental harm to the civilian population and civilian objects is unfortunate, but inevitable.
  Thus, the proportionality standard does not require that no incidental harm results from attacks. Rather, the
  combatants must determine whether, in engaging in offensive or defensive operations, his or her actions may
  be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a
  combination thereof, which would be excessive in relation to the concrete and direct military advantage
  anticipated to be gained (consider AP I art. 51(5)(b), 57(2)(b)). Under LOAC, judgments of proportionality
  often involve difficult and subjective comparisons. Recognizing these difficulties, States have declined to
  use the term proportionality in LOAC treaties because it could incorrectly imply that equilibrium or a precise
  comparison between considerations is possible (see DOD Law of War Manual, 2.4.1.2).
  Justification in Acting
  1-47. Proportionality is implicated in cases in which one is justified in acting. For example, an attack on a
  military objective that incidentally damages civilian property would raise proportionality considerations. On
  the other hand, where there is no justification for acting, such as an unlawful attack directed against the
  civilian population, a proportionality analysis would not be necessary to reach the conclusion that the attack
  was unlawful (see DOD Law of War Manual, 2.4.1.1).
  Excessive Harm
  1-48. Proportionality generally weighs the justification for acting against the expected harms to determine
  whether the latter is excessive compared with the former. In war, incidental harm to the civilian population
  and civilian objects is unfortunate and tragic, but inevitable. Thus, the proportionality standard does not
  require that no incidental harm results from attacks. Rather, combatants must refrain from attacks in which
  the expected harm of loss of civilian life, injury to civilians, damage to civilian objects, or a combination
  thereof, would be excessive in relation to the concrete and direct military advantage anticipated to be gained
  (see DOD Law of War Manual, 2.4.1.2; consider AP I art. 51(5)(b), 57(2)(b)). Under LOAC, judgments of
  proportionality often involve difficult and subjective comparisons. Often an equilibrium or a precise
  comparison between considerations is not possible.
  .
  CLASSES OF PERSONS
  1-49. This section addresses different classes of persons under LOAC. In the past, a citizen or national of a
  State at war, as one of the constituents of a State that is engaged in hostilities, could be subjected to the
  hardships of war by an enemy State (see “Lieber Code” arts. 21-24). As civilization has advanced and
  certainly today, however, consistent with the principles of military necessity and humanity, military
  operations generally may not be directed against civilians (see DOD Law of War Manual, 5.2.2). Thus, under