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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We are lifelong Zionists. Like other progressive Jews, our support for Israel has been founded on two convictions: first, that a state was necessary to protect our people from future disaster; and second, that any Jewish state would be democratic, embracing the values of universal human rights that many took as a lesson of the Holocaust. Undemocratic measures undertaken in pursuit of Israel’s survival, such as the occupation of the West Bank and Gaza and the denial of basic rights to Palestinians living there, were understood to be temporary.

But we must face reality: The occupation has become permanent. Nearly half a century after the Six-Day War, Israel is settling into the apartheid-like regime against which many of its former leaders warned. The settler population in the West Bank has grown 30-fold, from about 12,000 in 1980 to389,000 today. The West Bank is increasingly treated as part of Israel, with the green line demarcating the occupied territories erased from many maps. Israeli President Reuven Rivlin declared recently that control over the West Bank is “not a matter of political debate. It is a basic fact of modern Zionism.”

This “basic fact” poses an ethical dilemma for American Jews: Can we continue to embrace a state that permanently denies basic rights to another people? Yet it also poses a problem from a Zionist perspective: Israel has embarked on a path that threatens its very existence.

As happened in the cases of Rhodesia and South Africa, Israel’s permanent subjugation of Palestinians will inevitably isolate it from Western democracies. Not only is European support for Israel waning, but also U.S. public opinion — once seemingly rock solid — has begun to shift as well, especially among millennials. International pariah status is hardly a recipe for Israel’s survival.

At home, the occupation is exacerbating demographic pressures that threaten to tear Israeli society apart. The growth of the settler and ultra-orthodox populations has stoked Jewish chauvinism and further alienated the growing Arab population. Divided into increasingly irreconcilable communities, Israel risks losing the minimum of mutual tolerance that is necessary for any democratic society. In such a context, violence like the recent wave of attacksin Jerusalem and the West Bank is virtually bound to become normal.

Finally, occupation threatens the security it was meant to ensure. Israel’s security situation has changed dramatically since the 1967 and 1973 wars. Peace with Egypt and Jordan, the weakening of Iraq and Syria, and Israel’s now-overwhelming military superiority — including its (undeclared) nuclear deterrent — have ended any existential threat posed by its Arab neighbors. Even a Hamas-led Palestinian state could not destroy Israel. As six former directors of Israel’s internal security service, Shin Bet, argued in the 2012 documentary “The Gatekeepers,” it is the occupation itself that truly threatens Israel’s long-term security: Occupation forces Israel into asymmetric warfare that erodes its international standing, limits its ability to forge regional alliances against sectarian extremists and, crucially, remains the principal motive behind Palestinian violence.

In making the occupation permanent, Israel’s leaders are undermining their state’s viability. Unfortunately, domestic movements to avert that fate have withered. Thanks to an economic boom and the temporary security provided by the West Bank barrier and the Iron Dome missile defense system, much of Israel’s secular Zionist majority feels no need to take the difficult steps required for a durable peace, such as evicting their countrymen from West Bank settlements and acknowledging the moral stain of the suffering Israel has caused to so many Palestinians.

We are at a critical juncture. Settlement growth and demographic trends will soon overwhelm Israel’s ability to change course. For years, we have supported Israeli governments — even those we strongly disagreed with — in the belief that a secure Israel would act to defend its own long-term interests. That strategy has failed. Israel’s supporters have, tragically, become its enablers. Today, there is no realistic prospect of Israel making the hard choices necessary to ensure its survival as a democratic state in the absence of outside pressure.

For supporters of Israel like us, all viable forms of pressure are painful. The only tools that could plausibly shape Israeli strategic calculations are a withdrawal of U.S. aid and diplomatic support, and boycotts of and divestitures from the Israeli economy. Boycotting only goods produced in settlements would not have sufficient impact to induce Israelis to rethink the status quo.

It is thus, reluctantly but resolutely, that we are refusing to travel to Israel, boycotting products produced there and calling on our universities to divest and our elected representatives to withdraw aid to Israel. Until Israel seriously engages with a peace process that either establishes a sovereign Palestinian state or grants full democratic citizenship to Palestinians living in a single state, we cannot continue to subsidize governments whose actions threaten Israel’s long-term survival.

Israel, of course, is hardly the world’s worst human rights violator. Doesn’t boycotting Israel but not other rights-violating states constitute a double standard? It does. We love Israel, and we are deeply concerned for its survival. We do not feel equally invested in the fate of other states.

Unlike internationally isolated states such as North Korea and Syria, Israel could be significantly affected by a boycott. The Israeli government could not sustain its foolish course without massive U.S. aid, investment, commerce, and moral and diplomatic support.

We recognize that some boycott advocates are driven by opposition to (and even hatred of) Israel. Our motivation is precisely the opposite: love for Israel and a desire to save it.

Repulsed by the Afrikaners’ ethno-religious fanaticism in South Africa, Zionism founder Theodore Herzl wrote, “We don’t want a Boer state, but a Venice.” American Zionists must act to pressure Israel to preserve Herzl’s vision — and to save itself.