The fact that hundreds of Palestinians have been killed during the operation in Gaza, compared to fewer than 20 Israelis, has nothing to do with the question of whether the operation is legal according to international law, says Prof. Yuval Shany, an expert in international law from Hebrew University's law faculty.

The relevant question, he said, is "whether the operation is proportionate to the provocation that led to it. When a single Qassam [rocket] is fired, the state cannot invade and conquer an entire country. There must be a measure of proportion between the action and the reaction. But here, we are not talking about a single Qassam, but about years of Qassams."

Israel, he continued, "is permitted to use force to the degree necessary to end the attacks against it. Therefore, it [the operation] is legal as long as it is meant to prevent the attacks."

However, Shany stressed, by law, Israel would not have the right to use force to effect regime change in the Gaza Strip. Israel would also have no right to deliberately target Palestinian civilians, even though Hamas deliberately targets Israeli civilians: One side's illegal actions do not entitle the other side to violate the law as well.

"In wartime, it is permissible to attack military targets only," Shany explained. "This means targets that make a significant contribution to the other side's war effort: Qassam launchers, Hamas fighting forces, weapons storehouses and [smuggling] tunnels."

Military targets can be struck even if civilians will very likely be hurt, as long as the harm to civilians is proportionate, he explained. This depends on factors such as the military value of the target, the extent of the harm suffered by civilians and the measures taken to minimize this harm.

Thus, with regard to two specific dilemmas faced by Israel - whether to attack mosques being used as weapons storehouses, and also hospitals where senior Hamas commanders are holed up - Shany said: "A mosque is a more acceptable target than a hospital, because with a hospital, the assumption is that the harm to civilians will be far greater." And in fact, Israel has chosen to strike mosques, but not hospitals.

However, the professor added, even a hospital does not have total immunity: Firing missiles at it would be unacceptable, but a commando force could be sent in to capture wanted Hamas men.

Regarding claims that Israel has deprived Gaza of fuel and electricity, and prevented the evacuation of the wounded, Shany said that once Israel has taken control of the Strip, it must enable the population's humanitarian needs to be met. This includes an obligation to treat the wounded and to supply food, water and electricity. "The longer the army remains in an area, the greater its obligation to supply the local population's needs becomes," he added.

Similarly, when Israel warns civilians to leave a house before an attack, it must ensure that they have somewhere to go and access to basic necessities such as food and water. Nevertheless, Shany noted, when United Nations agencies examined Israel's conduct during the Second Lebanon War in 2006, they praised its practice of dropping leaflets to warn civilians to leave before bombing, saying this reduced civilian casualties.

Shany said it would be hard for anyone to sue Israel in the International Court of Justice in the Hague, since the country being tried must consent to have the court hear the case. However, he warned, individual European countries that claim universal jurisdiction could seek to arrest and try specific Israel Defense Forces officers for alleged war crimes.