Israel Ziv, a retired Israeli Army general, was taking a bike ride Saturday morning when a flood of alarming calls started coming in.

A huge barrage of rockets had been fired from Gaza. Gunmen from Hamas, the armed Palestinian group that controls the territory, were pouring across the border. Soon he would learn a friend’s son was trapped in a kibbutz.

He raced home, put on his uniform and grabbed his weapon, a nine-millimeter pistol.

Within minutes he was flying down a deserted highway in his new white Audi. As he neared the Gaza border, columns of black smoke rose in front of him, and the Israeli Army, at least at first, was nowhere to be seen. Hamas attackers were running across the landscape, hunched under the weight of heavy machine guns and rocket propelled grenade launchers, shooting at him.

“They were all over,” he said. “Hundreds of them.”

Mr. Ziv, stocky, spiky-haired, a bit irascible, and the former head of the operations directorate of the Israel Defense Forces, is a well-known figure in Israel, especially now. His actions over the weekend — driving headlong into the battle zone armed only with a pistol, organizing a confused group of soldiers into a fighting unit and overseeing evacuations — have been widely publicized on Israeli news channels. In the process he has become an avatar of Israel’s D.I.Y. spirit — and of the failure of its military and intelligence agencies.

The Israeli government said the toll in the devastating incursion by Hamas had reached 1,200 people killed, most of them unarmed civilians.

Already, amid the anguish over the slaughter, public frustrations are beginning to boil, with many Israelis, Mr. Ziv among them, taking issue with the government of Prime Minister Benjamin Netanyahu.

“The government is totally paralyzed,” said Mr. Ziv, who, even before this crisis, was extremely critical of Mr. Netanyahu for what he said were policies that bitterly divided Israelis and put the country’s security at risk.

Nevertheless, Mr. Ziv is still welcome in Israel’s corridors of power. On Wednesday, he held several teleconferences with captains of industry about raising tens of millions of dollars to help victims and their families.

“Just for civilians,” he shouted into his phone. “None of it for the army.”

He spoke to the top brass of the military and the police about shoring up a civilian defense force that had clearly been overwhelmed.

He even walked into Israel’s Defense Ministry, where he met with the defense minister, Yoav Gallant, and held secret meetings with national security officials in which they left their mobile phones on the hallway floor before stepping inside a small office for a chat that, the hope was, could not be tracked.

So weakened is public faith in the country’s military that one of the biggest issues Israelis are talking about is arming themselves. Many already own weapons, but the government announced this week that it was purchasing 10,000 assault rifles for civilians, along with bulletproof vests. Mr. Ziv is spearheading an effort to empower retired generals and former soldiers to rebuild community defense squads in the Gaza border area and around the country.

“We need weapons,” one man pleaded with Mr. Ziv as he visited a massacre site on Wednesday. “And we need a system.”

Mr. Ziv put a hand on the man’s back and said, “We are putting together that system right now.”

As they spoke, huge booms thundered and black smoke billowed up from the horizon, obscuring the banana farms and the wire fence along Gaza’s border with Israel that Hamas had breached to launch the assault. Gaza, only a few miles away, has been under relentless attack by Israeli warplanes since Saturday, killing hundreds of Palestinians.

And in just about every village where Israelis have been slaughtered, when a light breeze stirred the slender eucalyptus trees it also carried the smell of death.

Mr. Ziv spent Wednesday moving through this landscape. Sixty-six years old and a decorated paratrooper, he revisited the same terrain where he had tried to rescue as many people as he could. That included the site of the ill-fated desert rave party where Hamas terrorists massacred hundreds of young people — which Mr. Ziv believes might have been a primary target of the attack. Just about everywhere he went, soldiers and civilians thanked him, then shyly asked for a selfie.

His account of what he did on Saturday has been backed up by other retired generals and active duty officers who fought with him over the weekend.

He left his house, a beautiful home overlooking olive groves near Tel Aviv, and arrived in the battle zone around 10 a.m. He was traveling with a close friend, Noam Tibon, a retired general whose son was trapped in the Nahal Oz kibbutz.

Mr. Tibon’s son, a prominent journalist, had called his father in deep distress, saying gunmen were closing in on him and his family. In recent media interviews, Mr. Tibon said he told his son: “Trust me, I will come. This is my profession. Nobody can stop me.”

Mr. Ziv said that as they drove closer to Gaza, fires burned everywhere and unchallenged Hamas gunmen fired into buildings and passing cars. At first, he said, he didn’t see any Israeli soldiers. But as they traveled deeper toward the besieged villages, they encountered small bands of Israeli soldiers trying to fight back but clearly outnumbered.

“Things were not organized,” Mr. Ziv said.

He and Mr. Tibon linked up with a platoon of young soldiers, piled several of them into the Audi and began attacking Hamas gunmen on the road, Mr. Ziv said.

It was difficult taking them on with just a pistol, Mr. Ziv said, but after a soldier in his car was wounded, Mr. Ziv snatched his M16 and started firing out the window.

The worst feeling, though, was knowing that although they were some of the first responders, they were already too late.

Bodies were strewn on the highway, along the paths in the kibbutzim, in the patches of shaded forest they passed. What Mr. Ziv shared has been corroborated by extensive video and photo evidence, some of it filmed by the Hamas gunmen themselves. They hunted down Israeli civilians sitting in their cars, huddling in their homes, hiding at a bus stop and running for their lives.

“No one could imagine they would do what they did,” Mr. Ziv said. “It is a brutality that we have not witnessed since the establishment of Israel.”

He added, “So now we need to change the whole doctrine about Gaza. No more Hamas.”

How do you do that? he was asked.

“Level the ground,” he said.

Mr. Ziv and Mr. Tibon split up near the kibbutz where Mr. Tibon’s son lives. While Mr. Tibon joined a group of Israeli soldiers fighting Hamas members there, and eventually rescued his son, Mr. Ziv raced to other hot spots. He said he spent nearly 24 hours straight rushing around the kibbutzim and villages under attack, firing his own weapon, organizing evacuations of civilians and coordinating with the military to dispatch backup units as fast as possible.

The worst he found was the rave site. On Friday night, several thousand young people, Israelis and many foreigners, had flocked to an open field a few miles from the Gaza border to hold an overnight open-air dance party. By the time Mr. Ziv reached it Saturday night, he said, there was nothing left to be done.

There were bodies everywhere: in the campsite; in the field where everyone had been dancing; in car after car after car lining the road, filled with young people trying to escape.

He ran to one young man slumped out of a car and felt his neck. No pulse.

“I think the trigger for this whole attack was this event,” Mr. Ziv said. “Hamas planned this for a long time. But they knew a critical mass would be here this weekend.”

From evidence the Israeli military found at the rave site, and what witnesses said, the Hamas attackers surrounded the gathering on three sides. One group of gunmen opened fire on the crowd, methodically driving the panicked partygoers toward the road, where more gunmen were waiting to mow them down.

“I can still hear them screaming,” Mr. Ziv said.

He stood on the site looking out at a field littered with water bottles, rolled up sleeping mats, still-full boxes of Oreos, shirts, pants, tents and empty camp chairs. It was like everything was there but the people. One soldier quietly moved past him, carrying a black plastic bag, looking for documents.

“People don’t understand how fragile the situation is,” Mr. Ziv said. “Hamas has to pay for this.” He paused. “With their existence.”

He then walked away.

Eli Garshowitz contributed reporting from Be’eri, Israel.

The New York Times