The civilian death toll in Gaza from Israel’s latest incursion is appalling. The right to self-defense is inalienable, but it is not free from moral constraints.

As of this writing, nearly 750 Palestinians, including dozens of children, have been killed since the Israeli assault began. On Thursday, a compound housing a United Nations school — crowded with Gaza residents who had fled their homes to seek shelter — was shelled in an incident still under investigation by the Israeli Defense Forces. Palestinian officials said 15 people were killed and scores injured.

I support Israel. I abhor Hamas. But unleashing such devastating firepower on a tiny, densely crowded enclave in which civilians are trapped — and thus destined to become casualties — is wrong by any reasonable moral standard.

The Israeli government’s motivations in Gaza deserve to be taken seriously. But they do not justify the onslaught that is now in its third week. For Israeli military action to be justifiable, it must be proportionate. What we’re witnessing is not.

Israeli Prime Minister Benjamin Netanyahu said Sunday that Hamas is “targeting civilians and hiding behind civilians,” which he called a “double war crime.” He was referring to the fact that Hamas targets Israeli civilians with its rocket attacks and positions its military installations in residential neighborhoods or near schools and hospitals.

Netanyahu is right that these practices are reprehensible and that Israel has every right to respond. But none of this absolves Israel from its own moral responsibility. A civilized nation does not repay every heinous act in kind.

Israel says it is taking great pains to avoid civilian deaths. Indeed, Israel has been warning people to leave — with leaflets, text messages and nonlethal “roof-knocking” bombs — before smashing into residential neighborhoods. It is also true that in many instances Hamas, even knowing that an attack was coming, has instructed Gazans to stay put.

I have seen no confirmed reports, however, of Hamas using force to keep people in targeted areas so they can serve as human shields — and perhaps sway world opinion by boosting the body count. When people decide they must leave their homes, they can do so. But where are these evacuees supposed to go? To the nearest school or hospital? Not if these, too, are considered legitimate targets by the Israeli Defense Forces.

Gazans cannot flee across the closed border with Egypt. They obviously do not have the option of escaping into Israel or sailing away across the Mediterranean Sea. Gaza’s 1.8 million people are packed into an enclave measuring 139 square miles — an area and population roughly the size of Philadelphia.

Israeli officials say they would never consider attacking such targets as a school, a hospital or an apartment building unless Hamas were using these places — which should be off-limits in war — as military command posts, launch sites for rocket attacks and entry points for tunnels through which assassins and suicide bombers could enter Israel.

Again, however, there is the issue of proportionality. The military and political leadership of Hamas has much better intelligence about what the Israeli armed forces are doing and more options for refuge and shelter than the average Gazan. Indeed, we have not heard of any major Hamas figure being killed. So if you’re an Israeli commander and you know that there’s a Hamas military facility next to a medical clinic, but you’re not completely sure the militants are still there, while the clinic is likely packed with injured civilians, do you still pull the trigger?

Hamas’s rockets are much less of a threat than in the past because of Israel’sIron Dome missile-defense system, which has an impressive record. The tunnels are more worrisome because of their potential for use in future terrorism. Assume for the sake of argument that Israel had no choice but to act. What, then, would be a morally acceptable number of Palestinian civilian casualties?

Let me frame the question in practical terms: How many civilian casualties are needed to guarantee another generation of hatred and war?

The scale of death and destruction appears to be aimed not just at lessening the actual threat from Hamas but also at punishing Gazans for elevating Hamas to power in the first place. Netanyahu seems determined to teach them a lesson.

From all reports, however, the people of Gaza were already weary of Hamas. Netanyahu could have offered them an alternative future of free movement, economic development and peace. Instead, he gives them no choice.

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