Palestinian Arabs, by their first use of armed force against Israeli civilians and non-combatant Jews, in contravention of the United Nations Charter, constitute prima facie [Latin: on its face] 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 aggressors to comply with its international obligations. Israelís countermeasures need not be the exact equivalent of the breaching act.
Judge Schwebel, the former president of the International Court of Justice is quoted as 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."