It would be a denial of reality to assume that only a small group of terrorists could be involved in the thousands of documented acts of terror in the past 10 years by Palestinian Arabs. In the seven years since Israel withdrew from Gaza in 2005, Hamas has launched thousands of rockets directly at civilian populations in Israel. It would be reckless to assume that these acts of terror are isolated, undertaken independently, without the direct involvement of the Palestinian populace and the leadership they opt to elect.
Israel often is portrayed in the media, by Western leaders, human rights activists and the many different organs of the United Nations as inflicting disproportionate and collective punishment on many Palestinians for the deeds of a few terrorists.
Ironically, the prohibition of imposing "collective penalties [punishment] ... intimidation and terrorism" that Article 33 of the Fourth Geneva Convention  talks about, should be applied in this case to the millions of innocent men, women, and children of Israel that are collectively being punished day-in and day-out by Arab terrorists for "offenses" they never "personally committed." As to the "terrorism" Article 33 speaks about; it is the Israelis that fight to prevent Palestinian terrorism, and not the other way around.
Interestingly, the British in 1929 thought that collective punishment was a perfectly legitimate measure when inhabitants of Arab villages attacked the Jews. This collective punishment was not merely a single necessary step, but actually an existing ordinance of the British Mandate supported by the League of Nations, in dealing with Arab terrorism:
Collective Punishments. The Collective Punishments Ordinances were applied to the [Arab] towns and villages whose inhabitants were guilty of participation in the concerted attacks on Jews at Hebron, Safad, Motza, Artuf, Beer-Tuvia, and heavy fines were inflicted." 
Throughout history, Jews were law-abiding, peaceful people defending themselves against Arab aggression. In a 1946 Report, the Anglo-American Committee described its observation regarding Jews living in the land of Palestine:
"The Jew had to train himself for self-defence, and to accustom himself to the life of a pioneer in an armed stockade. Throughout the Arab rising, the Jews in the National Home, despite every provocation, obeyed the orders of their leaders and exercised a remarkable self-discipline. They shot, but only in self-defence; they rarely took reprisals on the Arab." 
Israel's reaction to Arab aggression is nothing more than a measured, fair response designed "to effectively terminate the attack [s]" by a conglomerate of Palestinian Arab terrorists, supported by Iran, Syria, and Hezbollah, in order to prevent its recurrence. 
Palestinian Arabs, by their first use of armed force against Israeli civilians and non-combatant Jews in contravention of the United Nations Charter, constituted 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. Israel countermeasures need not be the exact equivalent of the breaching act.  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." 
The perception among Palestinians that politically motivated violence is legitimate and effective is nothing new. From a broader perspective, if the Palestinians are rewarded with political gains following their acts of aggression, it can be expected that other radical groups will also make use of their tactics. Israel will no longer be the main target.
 See: http://www.icrc.org/ihl.nsf/c525816bde96b7fd41256739003e636a/72728b6de56c7a68c12563cd0051bc40?OpenDocument
 From the report by His Majestys Government in the United Kingdom of Great Britain and Northern Ireland to the Council of the League of Nations on the Administration of Palestine and Transjordan for the year 1929.
 Report of the Anglo-American Committee of Enquiry regarding the problem of European Jewry and Palestine. Lausanne, 20th April, 1946.
 See Harvard Journal of Law & Public Policy; 3/22/2002; Beard, Jack M.
 See UN GA Resolution 3314.
 United States Department of State, Draft Articles on State Responsibility, Comments of the Government of the United States of America, March 1, 2001. See: http://www.state.gov/documents/organization