Claiming that Israel “indiscriminately, callously, carelessly or indifferently” killed civilians in Gaza last summer is “nonsense,” an author of Israel’s military code of ethics told an audience late last month at the Jewish Theological Seminary of America.

“It’s not a serious account of what happened,” said Moshe Halbertal, a law professor at New York University and a professor of Jewish thought and philosophy at Hebrew University.

But while praising the Israel Defense Forces’ behavior in Gaza, Halbertal aimed some harsh words at the government of Benjamin Netanyahu, saying that it hasn’t done all it can do to avoid conflict, either in Gaza or on the West Bank. He also said that his views are shared by Israel’s national-security professionals — leaders of the Shin Bet, the Mossad and military intelligence — who believe in reopening negotiations with the Palestinian Authority.

Halbertal presented his views before an audience of about 300 people Nov. 20 during a conversation with Arnold Eisen, the seminary’s chancellor, who spoke of his guest’s contributions to Israeli society and world Jewry in glowing and even personal terms.

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Eisen cited a lengthy article Halbertal wrote in response to the 2009 Goldstone Report, a United Nations paper sharply condemning Israel’s actions in the first Gaza war, saying he “needed that voice to know what a person like me would make of the situation.” As for the IDF’s code of ethics, the evening’s main theme, the chancellor said they were “not the abstract musings of a philosopher,” but the “operative policy directing every Israeli soldier and in which every Israeli soldier is trained.”

Before discussing Gaza, Halbertal outlined for his listeners the three basic principles on which the code is based. They start with “necessity,” meaning that an army or individual soldier can apply force only for the purpose of the mission. Soldiers, for instance, can break into a home to conduct a search, but they can’t “break the TV,” which has nothing to do with the mission.

The second principle is “distinction,” which instructs soldiers that their firepower can only be aimed at those who pose a threat, either to the soldiers themselves or to civilians. Targets can include anyone who’s part of what Halbertal called the “causal chain,” including the planner, the person who recruits the terrorist and the one who builds the bomb, but they don’t include civilians who cheer the action, give moral support to the terrorists or engage in propaganda.

The third principle is “responsibility,” Halbertal continued — the most important one in the sort of asymmetrical warfare Israel now faces, he believes. Under that principle, soldiers know that the war is bound to cause “collateral harm to civilians,” but they have to do whatever they can to minimize that harm — even to the point of “assuming a calculated risk” to their comrades in arms.

A fourth, but more complicated, principle discussed by Halbertal is “proportionality,” which he suggested is overused and often misunderstood. That principle requires soldiers to ask themselves if the collateral harm that may be generated by their action is proportionate to the military achievement they hope to achieve.

Describing one application, Halbertal raised the possibility of a sniper on a rooftop. “The only way of incapacitating him is with a missile that would destroy the whole house, but you know there are 20, 30, non-combatants in the house.” In that case, he said, the value of killing the military target — the sniper — isn’t proportionate to the killing of 30 civilians.

Turning to how those principles were applied in Gaza, Halbertal said he thought about the matter and “struggled” with it while the war was being waged. Mistakes certainly took place, just as they do in any war, and some were caused by negligence, panic or simply not caring enough, he said, while two “systemic” issues troubled him even more. But overall, he said, the IDF did a great job in following its own code of ethics.

Halbertal bases that conclusion on several facts, including the ratio of combatants to non-combatants in Gaza. Roughly 30,000 Hamas and Islamic Jihad combatants reside in Gaza in a population of 1.8 million, Halbertal said — a 1-to-60 ratio. Other factors include the density of Gaza’s population and the reality that combatants in Gaza are more protected than civilians.

“It’s hard to talk numbers because you’re talking about human lives,” Halbertal said, but numbers still offer the best tool for measuring whether the IDF observed its own principles. Of the 2,000 Gaza residents killed in Gaza, he added, 700 to 900 were combatants — a figure that would have been closer to 20 combatants out of the 2,000 total “if Israel attacked Gaza the way the Allies [during World War II] attacked Dresden.”

“For an army to go from an initial ratio of 1 to 60” — the number of combatants to non-combatants living in Gaza — “to a ratio of 1 to 2” suggests one thing, Halbertal concluded: that it made a real effort to target only combatants and to minimize collateral harm. Other armies in situations a lot less complex — NATO forces in Kosovo, for instance, or American forces in Iraq — haven’t done as well, he said.

The two systemic issues that trouble Halbertal were the IDF’s policy of targeting the homes of Hamas officers — a practice he believes makes it much tougher to avoid civilian deaths — and a protocol in which the army pounds an area with artillery if a soldier has been kidnapped or is believed to have been kidnapped. Both, he believes, need to be studied seriously and re-examined.

Halbertal saluted the Netanyahu government for showing restraint in Gaza against the “temptation” of seizing the territory or “finishing off” Hamas, as advocated by Israelis further to the right. But that’s the only area in which he praised the prime minister.

“When you know that you’ll be engaged in a war in which, despite all the efforts you’ve made, there’ll be a lot of civilian deaths, you are responsible to do as much as you can to avoid it, if you can,” Halbertal said.

In Gaza, what that means for Halbertal is lifting the economic blockade — a policy he believes is “morally wrong and politically counterproductive” — and letting Gaza bloom. “Let them have something to lose,” he said. “Don’t corner them.”

It also means seriously engaging Mahmoud Abbas in peace talks, Halbertal said. He’s unsure that any agreement will result, he added, but he believes West Bank settlements and the “Har HaBayit,” or Temple Mount, have nothing to do with Israel’s interests. What does are matters of security.

Halbertal told his audience that he has vivid memories of the second intifada, especially the fear he felt each time he placed his children on a bus or went to a restaurant. He’s now concerned about the possibility of a third intifada, especially with “small-minded” Israeli politicians and Abbas fanning the flames, he said.

Responding to a question about the two-state solution, Halbertal dismissed suggestions that the option is dead, saying that there’s “no other option” for Israel. That option carries risks, he acknowledged, but the only alternative is allowing Israel to become a multiethnic state, like Syria, Lebanon and Iraq.

“You want to make Israel a multiethnic state?” Halbertal asked rhetorically. “This has a future? … You think that we — two strong national movements — can share space in a binational fantasy? It’s narishkeit [foolishness]. It’s nonsense.”

Halbertal advised his listeners to “listen carefully” to what Israel’s national-security professionals are saying. “These are people who don’t care about appeasing certain primary groups in their [political races].” Instead, he added, they’re looking at realities and searching for ways to address problems.

Halbertal speaks Thursday, Dec. 4, 7 p.m., at the JCC in Manhattan. For information, (646) 505-5708.