Questions of proportionality

Other incidents have raised concerns for these reasons, together with a second legal concept - proportionality.

This demands that the military gain of a particular operation be proportional to the likely or actual civilian losses incurred in carrying it out.

As Fred Abrahams, a senior researcher at Human Rights Watch puts it: "Even if you have a legitimate target you can’t just drop 10-tonne bombs on it."

Gaza medic rushes injured child into hospital
Questions of proportionality rest on intention as much as the numbers of people killed and injured

Five sisters in the Balousha family were killed as they slept together as, apparently, a nearby Hamas-linked mosque was bombed in Jabaliya refugee camp on the second day of Operation Cast Lead.

HRW is calling for an investigation. "Was the mosque a legitimate target? We have our doubts… Did they use weaponry that would limit damage to civilians? We have our serious doubts," says Mr Abrahams.

In this case, Capt. Rutland said the IDF had no record of a target in that specific area at that time, and gave no further explanation for the girls’ deaths.

A further case is the bombing of a truck that Israel initially said was loaded with missiles.

B’Tselem and the truck's owner – who said his son died along with seven other people – later said it was carrying oxygen canisters for welding. Israel maintains the warehouse the canisters were loaded from had been known to house weapons in the past.

How good was Israel's intelligence? How likely was it, for example, that at the moment of decision, the information might turn out to be wrong? And did the potential gains outweigh the possible losses?

Professor Sands says proportionality is "very, very difficult."

"What's proportionate in the eyes of one person may be disproportionate in the eyes of another," he says.

The difference in numbers in the Gaza war is stark - Palestinians say more than 500 Gazans have died in eight days, compared with 18 Israelis from rocket fire since 2001.

But experts say issues ranging from the parties' intentions, the reasons for going to war, the actions taken to protect - or indeed expose - civilians, and the conditions on the ground, all feed into a much more complicated legal equation.

Israel says lawyers are constantly consulted in its operations. It says it takes all possible steps to minimise civilian casualties.

Guided weapons are used; telephone warnings are often given before buildings are bombed; the IDF says missions have been aborted because civilians were seen at the target.

And it says its enemy is far from a standard army: "We're talking about an entire government whose entire raison d’etre is the defeat of Israel … and all of whose energies are directed at attacking Israeli civilians," says Capt. Rutland.

Witnesses and analysts confirm that Hamas fires rockets from within populated civilian areas, and all sides agree that the movement flagrantly violates international law by targeting civilians with its rockets.

But while B’Tselem's Ms Montell describes the rocket fire as a "blatant war crime", she adds: "I certainly would not expect my government to act according to the standard Hamas has set for itself - we demand a higher standard."