http://en.wikipedia.org/wiki/Fourth_Geneva_Convention#Collective_punishmentsCollective punishmentsArticle 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.
Pillage is prohibited.
Reprisals against protected persons and their property are prohibited. Under the 1949 Geneva Conventions collective punishments are a war crime. By collective punishment, the drafters of the Geneva Conventions had in mind the reprisal killings of World Wars I and World War II. In the First World War, Germans executed Belgian villagers in mass retribution for resistance activity. In World War II, Nazis carried out a form of collective punishment to suppress resistance. Entire villages or towns or districts were held responsible for any resistance activity that took place there. Additional concern also addressed the United States' atomic bombings of Hiroshima and Nagasaki, which, in turn, caused death and disease to millions
of Japanese civilians as well as their decedents. The conventions, to counter this, reiterated the principle of individual responsibility. The International Committee of the Red Cross (ICRC) Commentary to the conventions states that parties to a conflict often would resort to "intimidatory measures to terrorize the population" in hopes of preventing hostile acts, but such practices "strike at guilty and innocent alike. They are opposed to all principles based on humanity and justice."
Additional Protocol II of 1977 explicitly forbids collective punishment. But as fewer states have ratified this protocol than GCIV, GCIV Article 33 is the one more commonly quoted.