Op-Ed: Combatting Terrorism from Gaza: the Legal Issue
United Nations Security Council (UNSC) resolutions and International treaties define crimes against humanity as the "participation in, and knowledge of, a widespread or systematic attack against a civilian population, and the multiple commissions of [such] acts ... against any civilian population."
These international binding instruments go-on to impose uniform mandatory counter-terrorist obligations on all states to "bring to justice the perpetrators, organizers, and sponsors of terrorist attacks and stresses that those responsible for aiding, supporting or harboring the perpetrators, organizers and sponsors of these acts will be held accountable.
There is no escape clause - UNSC repeats "its unequivocal condemnation of all acts, methods and practices of terrorism as criminal and unjustifiable, regardless of their motivation, in all their forms and manifestations, wherever and by whomever committed."
Terrorist organizations such as Hamas that kidnap Israelis, blow up and destroy public buses, murder young people at cafés and discos, and school outings, alongside indiscriminate around-the-clock shelling of hundreds of Katyusha, Qassam, and Grad rockets targeting civilian populations in Israel, are clearly committing Crimes Against Humanity - acts of terrorism for which the UNSC under Chapter VII of the UN Charter directs everyone to fight terrorism by all means.
International law leaves no room to question Israel's right to defend its citizens against systematic and sustained terrorist attacks launched from Gaza, by Hamas. Israel's reaction to aggression is nothing more than a measured, fair response designed “to effectively terminate the attack [s]” upon it by terrorists supported by Iran, Syria, and Lebanon, in order to “prevent its recurrence.” Israel is far from using all means in fighting terrorism, as required by UNSC.
What unites Palestinian Arabs is their opposition to Jewish nationalism and a desire to stamp it out - not aspirations for their own state. Murdering Jews is a collective Palestinian Arab effort that requires a collectively sensible response.
Israel's reaction to nearly ten years of shelling Israeli civilian population centers from the Gaza Strip is nothing more than a measured, fair response, designed to effectively terminate armed attacks and more importantly - to prevent its recurrence. All of Israel’s actions in this regard are supported by international law.
Hamas - a United States designated foreign terrorist organization, by their aggression and initial use of armed force against Israeli civilians and non-combatant Jews in breach of the United Nations Charter, constituted prima facie evidence of an act of aggression - aggression being defined by international law as "the most serious and dangerous form of illegal use of force."
Therefore, the rule of proportionality in this case of continuous aggression, needs to be met by Israeli acts that will induce the aggressor to comply with international obligations. A countermeasure need not be the exact equivalent of the breaching act.
United Nations Resolutions demand of states to combat terrorism and reaffirm their:
“unequivocal condemnation of all acts, methods and practices of terrorism as criminal and unjustifiable, regardless of their motivation, in all their forms and manifestations, wherever and by whomever committed.” 
Judge Schwebel, the former President of the International Court of Justice is quoted saying:
"In the case of action taken for the specific purpose of halting and repelling an armed attack, this does not mean that the action should be more or less commensurate with the attack."
1 See: UN Resolution 3314 (XXIX). Definition of Aggression.
2 United States Department of State, Draft Articles on State Responsibility, Comments of the Government of the United States of America, March 1, 2001. See: www.state.gov/documents/
3 See: UNSC 1269, 1368, 1373, 1377