At exactly 1 p.m. on Oct. 7, 2023, Israel’s military leadership issued an order that unleashed one of the most intense bombing campaigns in contemporary warfare.

Effective immediately, the order granted midranking Israeli officers the authority to strike thousands of militants and military sites that had never been a priority in previous wars in the Gaza Strip. Officers could now pursue not only the senior Hamas commanders, arms depots and rocket launchers that were the focus of earlier campaigns, but also the lowest-ranking fighters.

In each strike, the order said, officers had the authority to risk killing up to 20 civilians.

The order, which has not previously been reported, had no precedent in Israeli military history. Midranking officers had never been given so much leeway to attack so many targets, many of which had lower military significance, at such a high potential civilian cost.

It meant, for example, that the military could target rank-and-file militants as they were at home surrounded by relatives and neighbors, instead of only when they were alone outside.

In previous conflicts with Hamas, many Israeli strikes were approved only after officers concluded no civilians would be hurt. Sometimes, officers could risk killing up to five civilians, and only rarely did the limit rise to 10 or above, though the actual death toll was sometimes much higher.

On Oct. 7, the military leadership changed its rules of engagement because it believed Israel faced an existential threat, according to a senior military officer who answered questions about the order on the condition of anonymity.

Hours earlier, Hamas-led terrorists had stormed into southern Israel, seizing towns and army bases, committing atrocities, firing thousands of rockets at civilian areas, killing up to 1,200 people and taking roughly 250 hostages. As Israelis battled Hamas fighters inside their borders, the officer said, Israel’s leaders also feared an invasion from the group’s allies in Lebanon and believed they had to take drastic military action.

An investigation by The New York Times found Israel severely weakened its system of safeguards meant to protect civilians, adopted flawed methods to find targets and assess the risk of civilian casualties, routinely failed to conduct post-strike reviews of civilian harm or punish officers for wrongdoing, and ignored warnings from within its own ranks and from senior U.S. military officials about these failings.

The Times reviewed dozens of military records and interviewed more than 100 soldiers and officials, including more than 25 people who helped vet, approve or strike targets. Collectively, their accounts provide an unparalleled understanding of how Israel mounted one of the deadliest air wars of this century. Most of the soldiers and officials spoke on the condition of anonymity because they were barred from speaking publicly on a subject of such sensitivity. The Times verified the military orders with officers familiar with their content.

In its investigation, the Times found that:

• Israel vastly expanded the set of military targets it sought to hit in preemptive airstrikes while simultaneously increasing the number of civilians that officers could endanger in each attack. That led Israel to fire nearly 30,000 munitions into Gaza in the war’s first seven weeks, more than in the next eight months combined. In addition, the military leadership removed a limit on the cumulative number of civilians its strikes could endanger each day.

• On a few occasions, senior commanders approved strikes on Hamas leaders that they knew would each endanger more than 100 noncombatants — crossing an extraordinary threshold for a contemporary Western military.

• The military struck at a pace that made it harder to confirm it was hitting legitimate targets. It burned through much of a prewar database of vetted targets within days and adopted an unproven system for finding new targets that used artificial intelligence at a vast scale.

• The military often relied on a crude statistical model to assess the risk of civilian harm and sometimes launched strikes on targets several hours after last locating them, increasing the risk of error. The model mainly depended on estimates of cellphone usage in a wider neighborhood, rather than extensive surveillance of a specific building, as was common in previous Israeli campaigns.

• From the first day of the war, Israel significantly reduced its use of so-called roof knocks, or warning shots that give civilians time to flee an imminent attack. And when it could have feasibly used smaller or more precise munitions to achieve the same military goal, it sometimes caused greater damage by dropping “dumb bombs” as well as 2,000-pound bombs.

The air campaign was at its most intense during the first two months of the war, when more than 15,000 Palestinians were killed — or roughly a third of the overall toll, according to the Gaza Health Ministry, which does not differentiate between civilians and combatants.

From November 2023 onward, amid a global outcry, Israel began to conserve ammunition and tighten some of its rules of engagement, including by halving the number of civilians who could be endangered when striking low-ranked militants who posed no imminent threat. But the rules remain far more permissive than before the war. Since those early weeks, more than 30,000 Palestinians have been killed, and while Israel disputes the ministry’s figures, the total continues to climb.

Provided a summary of the Times’ findings, the Israeli military acknowledged its rules of engagement had changed after Oct. 7 but said in a 700-word statement that its forces have “consistently been employing means and methods that adhere to the rules of law.”

The changes were made in the context of a conflict that is “unprecedented and hardly comparable to other theaters of hostilities worldwide,” the statement added, citing the scale of Hamas’ attack, efforts by militants to hide among civilians in Gaza, and Hamas’ extensive tunnel network.

“Such key factors,” the statement said, “bear implications on the application of the rules, such as the choice of military objectives and the operational constraints that dictate the conduct of hostilities, including the ability to take feasible precautions in strikes.”

Israel, which has been accused of genocide in a case before the International Court of Justice, says it complies with international law by taking all feasible precautions to minimize civilian casualties, often by ordering evacuations of whole cities before strikes and by dropping leaflets over neighborhoods and posting online maps about imminent operations.

Israel says Hamas’ military strategy makes bloodshed more likely. The group embeds itself in the civilian population, firing rockets from residential areas, hiding fighters and weapons inside homes and medical facilities, and operating from underground military installations and tunnels.

Unlike Hamas, which fires rockets indiscriminately at civilian areas, Israel and all Western armies operate under a multilayered oversight system that assesses the legality of planned strikes. Each attack plan is usually meant to be analyzed by a group of officers, which often includes a military lawyer who can advise on whether strikes might be unnecessary or unlawful.

To comply with international law, officers overseeing airstrikes must conclude the risk of civilian casualties is proportional to the target’s military value and take all feasible precautions to protect civilian life. But officers exercise significant discretion because the laws of armed conflict are vague about what counts as a feasible precaution or an excessive civilian toll.