Violating the rules of war:

A blatant disregard for international humanitarian law and the Geneva Convention.

Article 35.-Basic rules
paragraph 3. states It is prohibited to employ methods or means of warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment. In the case of threatening to retaliate in kind to weapons of mass destruction, this would seem to limit the use of atomic weapons to neutron bombs.

Collective punishments
Article 33. No protected person may be punished for an offense he or she has not personally committed. Collective penalties and likewise all measures of intimidation or of terrorism are prohibited.

Protocol III: Incendiary Weapons
Protocol III on Prohibitions or Restrictions on the Use of Incendiary Weapons prohibits, in all circumstances, making the civilian population as such, individual civilians or civilian objects, the object of attack by any weapon or munition which is primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat or a combination thereof, produced by a chemical reaction of a substance delivered on the target.

Article 2
Protection of civilians and civilian objects
1. It is prohibited in all circumstances to make the civilian population as such, individual civilians or civilian objects the object of attack by incendiary weapons.
2. It is prohibited in all circumstances to make any military objective located within a concentration of civilians the object of attack by air-delivered incendiary weapons.
3. It is further prohibited to make any military objective located within a concentration of civilians the object of attack by means of incendiary weapons other than air-delivered incendiary weapons, except when such military objective is clearly separated from the concentration of civilians and all feasible precautions are taken with a view to limiting the incendiary effects to the military objective and to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects.
4. It is prohibited to make forests or other kinds of plant cover the object of attack by incendiary weapons except when such natural elements are used to cover, conceal or camouflage combatants or other military objectives, or are themselves military objectives.

Geneva Protocol
1925 Geneva Protocol Prohibited the use of "asphyxiating gas, or any other kind of gas, liquids, substances or similar materials"

http://en.wikipedia.org/wiki/Geneva_Protocol
http://en.wikipedia.org/wiki/Geneva_Accord
http://en.wikipedia.org/wiki/Geneva_Conference_(1954)
http://en.wikipedia.org/wiki/Geneva_Conventions
International humanitarian law: the essential rules
http://www.icrc.org/Web/Eng/siteeng0.nsf/html/5ZMEEM