text: Conduct of Hostilities
  UNDEFENDED VILLAGES, TOWNS, OR CITIES
  Attack, by whatever means, of villages, towns, or cities that are undefended is prohibited (HR art.
  25; consider AP I art. 59). Undefended villages, towns, or cities may be captured, however.
  An undefended town, village, or city is any such inhabited place near or in a zone where opposing
  military forces are in contact with one another that contains no military objectives and is open for immediate
  physical occupation by enemy military forces without resistance. The term “undefended city” (or town or
  village, or any other populated area), sometimes referred to as an “open city,” does not include a city that has
  military objectives within it but lacks the means by which to defend them from attack (such as anti-air
  defenses) (HR art. 25).
  Although there is no explicit treaty requirement that a city be declared undefended before achieving
  that status, the practice has been to make such declarations to the opposing party (see DOD Law of War
  Manual, 5.15.3.2). Historically, a village, town, or city might seek undefended status as military forces
  advanced upon it and the opposing military forces present within it withdrew. Undefended status would
  essentially surrender the village, town, or city to the advancing force; this would minimize injury to the
  inhabitants and damage to civilian objects within the locality. This protection of civilian interests remains the
  primary purpose for designating a locality as undefended.
  An undefended town, village, or city may be established through negotiations with opposing forces,
  or unilaterally by the party to the conflict in control of it. If the latter, the intent and actions of that party
  should be communicated to opposing military forces through a declaration to avoid attack on the populated
  areas that are being designated as undefended. Belligerents may refuse to recognize a declaration that a city
  is undefended if they assess that it does not satisfy all of the necessary conditions, although they should notify
  the opposing belligerent of that decision. Absent or until acceptance, military objectives in the locality
  unilaterally designated as undefended remain subject to attack. Agreement to the undefended designation
  may be accomplished by a commander with the authority to enforce the corresponding obligations, such as
  an immediate military commander, theater commander, or by national level authorities.
  Conditions
  The area in question must be open and accessible for immediate, unconditional physical occupation
  by opposing military ground forces. The location may not be defended by artillery or naval gun fire from
  the flanks or rear, by aircraft, minefields, or any other form of military resistance. A party to the conflict
  may not declare a city or other population center to be undefended if it is far behind the area of ground
  conflict and impossible for opposing ground forces to occupy physically. For a populated area to be entitled
  to be regarded as undefended, each of the following criteria must be satisfied:
  * All opposing force combatants, as well as their mobile weapons and mobile military equipment,
  must have been evacuated.
  * No hostile use of fixed military installations or establishments within the city, town, or village
  may be made.
  * The local civilian authorities and the civilian population must not commit hostile acts against the
  occupying military force.
  * No activities in support of military operations may be undertaken.
  Loss of Status
  A village, town, or city that fails to fulfill the conditions above will not be entitled to undefended
  status, or, if granted, will lose that status if these conditions are not fulfilled. Persons and objects within that
  city, however, may still receive other protections for civilians and civilian objects.
  A locality designated as undefended does not lose that status due to the presence of civilian police
  forces to maintain local law and order, enemy military medical units and medical personnel, or enemy
  wounded and sick.