It was 3 a.m. on Oct. 7, and Ronen Bar, the head of Israel’s domestic security service, still could not determine if what he was seeing was just another Hamas military exercise.

At the headquarters of his service, Shin Bet, officials had spent hours monitoring Hamas activity in the Gaza Strip, which was unusually active for the middle of the night. Israeli intelligence and national security officials, who had convinced themselves that Hamas had no interest in going to war, initially assumed it was just a nighttime exercise.

Their judgment that night might have been different had they been listening to traffic on the hand-held radios of Hamas militants. But Unit 8200, Israel’s signals intelligence agency, had stopped eavesdropping on those networks a year earlier because they saw it as a waste of effort.

As time passed that night, Mr. Bar thought that Hamas might attempt a small-scale assault. He discussed his concerns with Israel’s top generals and ordered the “Tequila” team — a group of elite counterterrorism forces — to deploy to Israel’s southern border.

Until nearly the start of the attack, nobody believed the situation was serious enough to wake up Prime Minister Benjamin Netanyahu, according to three Israeli defense officials.

Within hours, the Tequila troops were embroiled in a battle with thousands of Hamas gunmen who penetrated Israel’s vaunted border fence, sped in trucks and on motorbikes into southern Israel and attacked villages and military bases.

The most powerful military force in the Middle East had not only completely underestimated the magnitude of the attack, it had totally failed in its intelligence-gathering efforts, mostly due to hubris and the mistaken assumption that Hamas was a threat contained.

Despite Israel’s sophisticated technological prowess in espionage, Hamas gunmen had undergone extensive training for the assault, virtually undetected for at least a year. The fighters, who were divided into different units with specific goals, had meticulous information on Israel’s military bases and the layout of kibbutzim.

The country’s once invincible sense of security was shattered.

More than 1,400 people were killed, including many women, children and old people who were murdered systematically and brutally. Hundreds are held hostage or are still missing. Israel has responded with a ferocious bombardment campaign on Gaza, killing more than 8,000 Palestinians and wounding thousands more, according to the Hamas-run health ministry. The Israeli military on Sunday signaled a heavier assault on Gaza, saying it had expanded its ground incursion overnight.

Israeli officials have promised a full investigation into what went wrong.

Even before that inquiry, it is clear the attacks were possible because of a cascade of failures over recent years — not hours, days or weeks. A New York Times examination, based on dozens of interviews with Israeli, Arab, European and American officials, as well as a review of Israeli government documents and evidence collected since the Oct. 7 raid, shows that:

• Israeli security officials spent months trying to warn Mr. Netanyahu that the political turmoil caused by his domestic policies was weakening the country’s security and emboldening Israel’s enemies. The prime minister continued to push those policies. On one day in July he even refused to meet a senior general who came to deliver a threat warning based on classified intelligence, according to Israeli officials.

• Israeli officials misjudged the threat posed by Hamas for years, and more critically in the run-up to the attack. The official assessment of Israeli military intelligence and the National Security Council since May 2021 was that Hamas had no interest in launching an attack from Gaza that might invite a devastating response from Israel, according to five people familiar with the assessments who spoke on the condition of anonymity to discuss sensitive details. Instead, Israeli intelligence assessed that Hamas was trying to foment violence against Israelis in the West Bank, which is controlled by its rival, the Palestinian Authority.

• The belief by Mr. Netanyahu and top Israeli security officials that Iran and Hezbollah, its most powerful proxy force, presented the gravest threat to Israel diverted attention and resources away from countering Hamas. In late September, senior Israeli officials told The Times they were concerned that Israel might be attacked in the coming weeks or months on several fronts by Iran-backed militia groups, but made no mention of Hamas initiating a war with Israel from the Gaza Strip.

• American spy agencies in recent years had largely stopped collecting intelligence on Hamas and its plans, believing the group was a regional threat that Israel was managing.

Overall, arrogance among Israeli political and security officials convinced them that the country’s military and technological superiority to Hamas would keep the terrorist group in check.

“They were able to trick our collection, our analysis, our conclusions and our strategic understanding,” Eyal Hulata, Israel’s national security adviser from 2021 until early this year, said during a discussion last week in Washington sponsored by the Foundation for Defense of Democracies, a think tank.

“I don’t think there was anyone who was involved with affairs with Gaza that shouldn’t ask themselves how and where they were also part of this massive failure,” he added.

Many senior officials have accepted responsibility, but Mr. Netanyahu has not. At 1 a.m. Sunday in Israel, after his office was asked for comment on this article, he posted a message on X, formerly Twitter, that repeated remarks he made to The New York Times and blamed the military and intelligence services for failing to provide him with any warning on Hamas.

“Under no circumstances and at no stage was Prime Minister Netanyahu warned of war intentions on the part of Hamas,” the post read in Hebrew. “On the contrary, the assessment of the entire security echelon, including the head of military intelligence and the head of Shin Bet, was that Hamas was deterred and was seeking an arrangement.”

In the resulting furor, Benny Gantz, a member of his war cabinet, publicly rebuked Mr. Netanyahu, saying that “leadership means displaying responsibility,” and urged the prime minister to retract the post. It was later deleted, and Mr. Netanyahu apologized in a new one.

On Sunday, Shin Bet promised a thorough investigation after the war. The I.D.F. declined to comment.

The last time Israelis’ collective belief in their country’s security was similarly devastated was 50 years earlier, at the start of the Yom Kippur War, when Israel was caught off guard by an assault by Egyptian and Syrian forces. In an echo of that attack, Hamas succeeded because Israeli officials made many of the same mistakes that were made in 1973.

The Yom Kippur War was “a classic example of how intelligence fails when the policy and intelligence communities build a feedback loop that reinforces their prejudices and blinds them to changes in the threat environment,” Bruce Riedel, a former top Middle East analyst at the Central Intelligence Agency, wrote in a 2017 research paper about the 1973 war.

In an interview this month, Mr. Riedel said that Mr. Netanyahu was reaping the consequences of focusing on Iran as the existential threat to Israel while largely ignoring an enemy in his backyard.

“Bibi’s message to Israelis has been that the real threat is Iran,” he said, using Mr. Netanyahu’s nickname. “That with the occupation of the West Bank and the siege of Gaza, the Palestinian issue is no longer a threat to Israel’s security. All of those assumptions were shattered on Oct. 7.”