I think that Hamas, Hezbollah, and Israel are all in violation of the Fourth Geneva Convention, Art. 33-34: “No protected person may be punished for an offence he or she has not personally committed. Collective penalties and likewise all measures of intimidation or of terrorism are prohibited. … Reprisals against protected persons and their property are prohibited. … The taking of hostages is prohibited.”
Hamas and Hezbollah took hostages, not just prisoners, if they captured soldiers for the purpose of exchanging them for Israeli prisoners. But Israel admits that its response is a form of collective punishment. For instance, “Israeli Prime Minister Ehud Olmert held Lebanon directly responsible for the ambush and promised a ‘painful and far-reaching response.'” The pain, evidently, is to be suffered by the Lebanese collectively, not just by Hezbollah as a military organization.
The Fourth Geneva Convention clearly binds Israel and Lebanon, which are signatories. When the PLO was responsible for the occupied territories, it filed a letter expressing a desire to sign as well. (The Swiss Federal Council replied “that it was not in a position to decide whether the letter constituted an instrument of accession, ‘due to the uncertainty within the international community as to the existence or non-existence of a State of Palestine.'”) In 1977, some countries–including the US but not Israel–accepted an addendum that would even more clearly render illegal some of the recent actions taken by Israel. I think the addendum is morally appropriate, although one could debate whether Hamas fits the description in Article 1 of “peoples fighting against colonial domination and alien occupation and against racist regimes in the exercise of their right of self-determination”.
The text of the addendum is below the fold. [I see that Chris Bertram on Crooked Timber has a similar take.]
This Protocol, which supplements the Geneva Conventions of 12 August 1949 for the protection of war victims, shall apply in the situations referred to in Article 2 common to those Conventions.
… The situations referred to in the preceding paragraph include armed conflicts which peoples are fighting against colonial domination and alien occupation and against racist regimes in the exercise of their right of self-determination, as enshrined in the Charter of the United Nations and the Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations. …
In addition to the grave breaches defined in Article 11, the following acts shall be regarded as grave breaches of this Protocol, when committed wilfully, in violation of the relevant provisions of this Protocol, and causing death or serious injury to body or health:
(a) making the civilian population or individual civilians the object of attack;
(b) launching an indiscriminate attack affecting the civilian population or civilian objects in the knowledge that such attack will cause excessive loss of life, injury to civilians or damage to civilian objects, as defined in Article 57, paragraph 2 (a)(iii);
(c) launching an attack against works or installations containing dangerous forces in the knowledge that such attack will cause excessive loss of life, injury to civilians or damage to civilian objects, as defined in Article 57, paragraph 2 (a)(iii);
(d) making non-defended localities and demilitarized zones the object of attack;
(e) making a person the object of attack in the knowledge that he is hors de combat …