Evidence that Israel committed war crimes in its 23-day operation in Gaza mounts by the week. Amnesty and Human Rights Watch have both appealed for a United Nations inquiry, after conducting their own investigations. Last week Ha'aretz published the testimonies of Israeli soldiers who alleged that a sniper shot a Palestinian mother and her two children, and that a company commander ordered an elderly woman to be killed. Yesterday Physicians for Human Rights accused soldiers of ignoring the special protection that Palestinian medical teams are entitled to receive. Today the Guardian releases three films in which our reporter Clancy Chassay reveals evidence that Israel used drones to fire at civilian targets, killing at least 48; he interviews three Palestinian youths used by Israeli soldiers as human shields and alleges that soldiers targeted paramedics and hospitals.

None of this is to deny that a case also exists against Hamas and other militant groups in Gaza. Firing unaimable rockets at civilians in southern Israel is also a war crime. But there is no symmetry of guilt. Israel has weapons it can place to within a metre of its intended targets. Its drones have high-quality optics that can see the colour of the target's sweater. And they film everything both before and after each attack. The army has the means to refute these allegations, but feels no obligation to do so. An international inquiry should be launched for no other reason than to hold it accountable.

Israel has not got a history of co-operating with international inquiries into the actions of its army, but it has reacted twice to domestic allegations. It admitted that one of its tanks fired two shells at the apartment of a Hebrew-speaking Palestinian doctor whose three daughters were killed and whose grief touched the nation, but it concluded that the action was "reasonable". The Ha'aretz material prompted a criminal inquiry by the military advocate, and two unusual statements by the outgoing defence minister, Ehud Barak, and the chief of staff, General Gabi Ashkenazi, each of whom praised the "moral" actions of the army. The prospects of a full international investigation of these allegations are mixed. The international criminal court has received more than 220 complaints from the Palestinian National Authority, the Arab League and the Palestinian justice minister. But whether the court has jurisdiction is another matter.

If the ICC route fails, there is always the UN, whose schools and stores found themselves in the line of fire. The secretary general, Ban Ki-moon, will this week receive the results of a private board of inquiry. This is narrow in scope, only examining incidents at UN facilities. But what happened there was bad enough, including the use of white phosphorus shells.

There are five reasons why we should have an international inquiry into the Israeli assault on Gaza. First, the conflict has not gone away. It could reignite at any moment under a prime minister, Binyamin Netanyahu, who is determined to finish the job. Second, the weight of evidence points not to isolated incidents, but to a new and deadly relaxation of the rules of engagement. This emerges from the soldiers' own testimony in Ha'aretz. "That's what is so nice, supposedly, about Gaza," one soldier said. "You see a person on a road ... He doesn't have to be with a weapon. You don't have to identify him with anything. You can just shoot him." Gaza was fought to a certain mood music. It suggested that the lives of Palestinian civilians did not matter when weighed against those of Israeli soldiers. Third, Israel is not immune to international opinion. A narrow rightwing coalition under Mr Netanyahu will be sensitive to criticism from Barack Obama, who has yet to reveal his cards. Fourth, what Israel does or is allowed to get away with doing affects attempts to establish the rule of international law in other conflicts. Fifth, we know what doing nothing leads to: another war, and ultimately a third intifada.