Over the course of 2023, before the attack, Israeli forces had killed 247 Palestinians, and Palestinians had killed 39 Israelis. Increases in settler attacks had displaced hundreds of Palestinians, and there were clashes around the Al-Aqsa Mosque, a contested holy site in Jerusalem.Tensions between Israel and Hamas rose in September 2023, and The Washington Post wrote that the two were "on the brink of war". On 13 September, five Palestinians were killed at the border. Israel said it found explosives hidden in a shipment and halted all exports from Gaza; Hamas denied this. Reuters quoted Palestinians who said that the several-day ban affected thousands of families. In response to the ban, Hamas put its forces on high alert and conducted military exercises with other groups, including openly practicing storming Israeli settlements. Hamas also allowed Palestinians to resume protests at the Gaza–Israel barrier. On 29 September, Qatar, the UN, and Egypt mediated an agreement between Israel and Hamas officials in the Gaza Strip to reopen closed crossing points and deescalate tensions; the total number of Gazans with work permits in Israel stood at 17,000.Egypt said it warned Israel days before the attack that "an explosion of the situation [was] coming, and very soon, and it would be big." Israel denied receiving such a warning, although Michael McCaul, Chairman of the US House Foreign Relations Committee, said that warnings were given three days before the attack.

Bedouin clans built smuggling tunnels on the Egypt–Gaza border in 1981. In 2001, Hamas began a vast underground network, initially for smuggling and later serving multiple functions. The tunnels aimed to shift battles underground. In 2014, Hamas employed 900 for tunnel construction, each taking three months and costing an average of $100,000. Funding came from commercial schemes via Gaza's mosques, with contributions from Iran and North Korea.Gaza's tunnel network, known euphemistically as the "Gaza metro", serves Hamas for storage, movement, and command. Hamas used hardwired phone lines within the tunnels for covert communication over two years, evading Israeli intelligence. This allowed a successful surprise attack on Israel, with specific plans disclosed shortly before the operation, catching intelligence agencies off guard.In the months preceding the attack, Hamas publicly released videos of its militants preparing to attack Israel. A video released in December 2022 showed Hamas training to take hostages, while another video showed Hamas practicing paragliding. On 12 September, Hamas posted a video of its fighters training to blast through the border. After the attack, the IDF said that Hamas had extensively studied the military bases and settlements near the border.The Wall Street Journal has accused Iran of being behind the attack. U.S. officials and Iran have denied this.The IDF has reported seizing over 10,000 weapons following the attack. The arsenal included RPGs, mines, sniper rifles, drones, thermobaric rockets, and other advanced weapons. According to Israeli sources, documents and maps seized from Hamas militants indicated that Hamas intended a coordinated, month-long operation to invade and occupy Israeli towns, cities, and kibbutzim, including attacking Ashkelon by sea and reaching Kiryat Gat, 20 miles into Israel. The scale of weapons, supplies, and plans indicated, according to Israel, that Hamas intended to inflict mass casualties on Israeli civilians and military forces over an extended period. Western and Middle Eastern security officials gathered evidence suggesting that Hamas intended to invade as far as the West Bank, had the initial attack been more successful.

According to The New York Times, Israeli officials had obtained detailed attack plans more than a year before the attack. The document described operational plans and targets, including the size and location of Israeli forces, and raised questions in Israel about how Hamas learned these details. The document provided a plan that included a large-scale rocket assault before an invasion, drones to knock out the surveillance cameras and automated guns that Israel has stationed along the border, and gunmen invading Israel, including with paragliders. The Times reported, "Hamas followed the blueprint with shocking precision." According to The Times, the document was widely circulated among Israeli military and intelligence leadership, who largely dismissed the plan as beyond Hamas's capabilities, though it was unclear whether the political leadership was informed. In July 2023, a member of the Israeli signals intelligence unit alerted her superiors that Hamas was conducting preparations for the assault, saying, "I utterly refute that the scenario is imaginary". An Israeli colonel ignored her concerns.According to Haaretz, Israel's domestic intelligence agency, Shin Bet, and IDF military commanders discussed a possible threat to the Nova music festival near kibbutz Re'im just hours before the attack, but the festival's organizers were not warned.According to a BBC investigation, surveillance reports suggested that Hamas was planning a significant operation against Israel, but senior IDF officers repeatedly ignored the warnings.

Deif said more than 5,000 rockets had been fired from the Gaza Strip into Israel in a span of 20 minutes at the start of the operation. Israeli sources reported the launch of 3,000 projectiles from Gaza, killing five. Explosions were reported in areas surrounding Gaza and in the Sharon Plain, including Gedera, Herzliyya, Tel Aviv, and Ashkelon. Air raid sirens were activated in Beer Sheva, Jerusalem, Rehovot, Rishon Lezion, and Palmachim Airbase. Hamas issued a call to arms, with Deif calling on "Muslims everywhere to launch an attack".Palestinian militants also opened fire on Israeli boats off the Gaza Strip, while clashes broke out between Palestinians and the Israel Defense Forces in the eastern section of the Gaza perimeter fence. In the evening Hamas launched another barrage of about 150 rockets towards Israel, with explosions reported in Yavne, Givatayim, Bat Yam, Beit Dagan, Tel Aviv, and Rishon Lezion.

Simultaneously, around 2,900 Palestinian militants infiltrated Israel from Gaza using trucks, pickup trucks, motorcycles, bulldozers, speedboats, and powered paragliders.The Sderot police station was reported to have come under Hamas control, with militants killing 30 Israelis, including policemen and civilians.

Early in the attack they deliberately destroyed the computer systems at the police station. This disabled communication and delayed the response to the attacks.Images and videos appeared to show heavily armed and masked militants dressed in black fatigues riding pickup trucks and opening fire in Sderot, killing dozens of Israeli civilians and soldiers and setting homes on fire. Other videos appeared to show Israelis taken prisoner, a burning Israeli tank, and militants driving Israeli military vehicles. Israeli first responders reportedly recovered documents from killed militants' bodies with instructions to attack civilians, including elementary schools and a youth center, to "kill as many people as possible", and to take hostages for use in future negotiations. Some of the militants wore body cameras to record the acts, presumably for propaganda purposes. According to reports, some militants used Captagon during the attacks—a stimulant produced in Syria and used throughout the Middle East.The morning of the attack, an Israeli military spokesman said that the militants from Gaza had entered Israel through at least seven locations and invaded four small rural Israeli communities, the border city of Sderot, and two military bases from both land and sea. Israeli media reported that seven communities came under Hamas control, including Nahal Oz, Kfar Aza, Magen, Be'eri, and Sufa. The Erez Crossing was reported to have come under Hamas control, enabling militants to enter Israel from Gaza. Israeli Police Commissioner Kobi Shabtai said there were 21 active high-confrontation locations in southern Israel.The New York Times reported that an Israeli intelligence document prepared weeks after the attack found that Hamas had breached the border fence in over 30 separate locations.Starting at 6.30 a.m. the same day, a massacre unfolded at an outdoor music festival near Re'im, resulting in at least 360 dead and many others missing. Witnesses recounted militants on motorcycles opening fire on fleeing participants, who were already dispersing due to rocket fire that had wounded some attendees; some were also taken hostage. Militants killed civilians at Nir Oz, Be'eri, and Netiv HaAsara, where they took hostages and set fire to homes, as well as in kibbutzim around the Gaza Strip. Around 50 civilians were killed in the Kfar Aza massacre, 108 in the Be'eri massacre, and 15 people in the Netiv HaAsara massacre. Militants killed 16 or 17 Thai and Nepalese employees during the Kibbutz Alumim massacre.Other Hamas militants carried out an amphibious landing in Zikim. Palestinian sources claim that the local Israeli army base was stormed. The IDF said it had killed two attackers on the beach and destroyed four vessels, including two rubber boats. Militants also attacked a military base outside Nahal Oz, leaving at least 18 dead and taking seven hostage. An IDF fire investigation found that the militants had "ignited substances... that contain toxic gasses which can cause suffocation within minutes, or even less" both at the base and in civilian locations.According to a December 2023 Ynet article, there was also an "immense and complex quantity" of friendly-fire incidents during the 7 October attack that "it would not be morally sound to investigate" given their number and the challenges soldiers were facing at the time. In January 2024, an investigation by Israeli newspaper Yedioth Ahronoth concluded that the IDF had in practice applied the Hannibal Directive, ordering all combat units to stop "at all costs" any attempt by Hamas terrorists to return to Gaza, even if there were hostages with them. It is unclear how many hostages were killed by friendly fire as a result of the order. According to Yedioth Ahronoth, around 70 burnt-out vehicles on roads leading to Gaza had been fired on by helicopters or tanks, killing all occupants in at least some cases.

Soon after the start of the Hamas operation, there were reports that many civilians and soldiers had been taken as captives back to the Gaza Strip. Later in the day Hamas announced it had captured enough Israeli soldiers to force a prisoner swap, and Israel confirmed hostages had been taken.In Be'eri, up to 50 people were taken hostage; after an 18-hour standoff between militants and IDF forces, they were freed. Hostages were also reported taken in Ofakim, where policemen led by Chief Superintendent Jayar Davidov engaged Palestinian militants in a shootout;[when?] Davidov and three of his men were killed, and the IDF later rescued two Israeli hostages in the suburb of Urim. There were reports of militants killing and stealing family pets.Hamas took many hostages back to Gaza. On 16 October, they said they were holding 250 hostages and that it had done so to force Israel to release its Palestinian prisoners. Some of the hostages, including three members of the Bibas family, were subsequently handed over to other militant groups. Palestinian Islamic Jihad ended up holding at least 30 of the hostages, but it is unclear whether they or Hamas originally kidnapped them.

According to Ariel Merari, the raiders "were ordered to kidnap as many [people] as possible... [and] they intentionally kidnapped a populace that is sensitive from the aspect of Israeli public opinion". Merari doubts that Hamas will agree to releasing all of the hostages in "one go" regardless of how many of its prisoners are released, since the hostages are its only guarantee against complete destruction at Israel's hands. He believes Hamas will try to force a ceasefire and protract the release for weeks or months, until an Israeli offensive is no longer seen as viable.

According to Chen Kugel, head of the Abu Kabir Forensic Institute, hundreds of bodies arrived at the institute in a state "beyond recognition". Pathologists were required to process, among others, bone fragments recovered from fires; a blood-soaked baby mattress; victims who were tied, then executed; and two victims who were tied, then incinerated alive.With hundreds missing and bodies burned beyond recognition, Israeli authorities assembled recovery teams from across society. This included archaeologists from the Israel Antiquities Authority, who identified and removed ancient remains in attempts to sift through ash and rubble for bone fragments other forensic teams overlooked.The sheer number of casualties overwhelmed authorities. Bodies were brought chaotically to the Shura IDF base and Abu Kabir forensic institute. The different military, police, and civilian teams caused confusion. Archeologists systematically searched rooms, dividing them into grids and carefully extracting bone shards. At one house, the archeology team found a bloodstain under ash that it determined was the outline of a body, later identified by DNA analysis as Meni Godard.

On 10 November, Israel revised its casualty count from 1,400 to 1,200 after concluding that many of the identified bodies were those of Hamas fighters. This included 859 civilians, 283 soldiers, 57 policemen, and 10 Shin Bet members.In December 2023, using social security data, this was further revised to 1,139: 695 Israeli civilians (including 36 children), 71 foreign nationals, and 373 security forces. Five people are classed as missing, including four Israelis.

After the initial breach of the Gaza perimeter by Palestinian militants, it took hours for the Israeli military to respond by sending troops to counterattack. The first helicopters sent to support the military were launched from the north of Israel, and arrived in Gaza an hour after fighting began. Israel had difficulty determining which outposts and settlements were occupied, and distinguishing between Palestinian militants and the soldiers and civilians on the ground. The helicopter crews initially poured down fire at a tremendous rate, attacking about 300 targets in four hours. Later, the crews began to slow their attacks and carefully select targets. According to Haaretz journalist Josh Breiner, a police source said that a police investigation found that an IDF helicopter that had fired on Hamas militants "apparently also hit some festival participants" in the Re'im music festival massacre. The Israeli police denied Haaretz's report.Subsequent investigation has determined that militants had been instructed not to run so that the air force would think they were Israelis. This deception worked for some time, but pilots began to realize the problem and ignore their restrictions. By around 9 a.m., amid the chaos and confusion, some helicopters started laying down fire without prior authorization. The attack appeared to have been a complete surprise to the Israelis.

The IDF launched Operation Swords of Iron in Gaza and declared a state of emergency for areas within 80 kilometers (50 mi) of the Gaza border. It also said that Hamas "made a grave mistake" in launching its attack and pledged that "Israel will win". The IDF declared a "state of readiness for war", adding that reservists were to be deployed not only in Gaza but also in the West Bank and along the borders with Lebanon and Syria. Residents in areas near Gaza were asked to stay inside, while civilians in southern and central Israel were "required to stay next to shelters". Roads around Gaza were closed by the IDF. Tel Aviv's streets were also locked down.After the attack, Israel declared a heightened state of preparedness for potential conflict. The IDF declared a state of readiness for war, and Netanyahu convened an emergency gathering of security authorities. The IDF additionally reported that it had begun targeted actions in Gaza under what it called Operation Swords of Iron (or Iron Swords) (Hebrew: מבצע חרבות ברזל, romanized: Mivtsá charvót barzél). Israeli Police Commissioner Kobi Shabtai announced that a "state of war" existed, following what he called "a massive attack from the Gaza Strip". He also announced the closure of all of southern Israel to "civilian movement" and the Yamam counterterrorism unit's deployment to the area. The IDF's chief spokesperson, Rear Admiral Daniel Hagari, said four divisions were deployed to the area, augmenting 31 preexisting battalions.Israeli President Isaac Herzog said the country was facing "a very difficult moment", and offered strength and encouragement to the IDF, other security forces, rescue services, and residents who were under attack. In a televised broadcast, Netanyahu said: "We are at war." He also said that the IDF would reinforce its border deployments to deter others from "making the mistake of joining this war". In a later address, he threatened to "turn Gaza into a deserted island" and urged its residents to "leave now".On 7 October, Israel's Security Cabinet voted to undertake a series of actions to bring about the "destruction of the military and governmental capabilities of Hamas and Palestinian Islamic Jihad", according to a statement by the Prime Minister's Office. The Israel Electric Corporation, which supplies up to 80% of Gaza's electricity, cut off power to the area. As a result, Gaza's power supply was reduced from 120 MW to 20 MW, forcing it to rely on power plants paid for by the Palestinian Authority.Ben Gurion Airport and Ramon Airport remained operational, but multiple airlines canceled flights to and from Israel. Israel Railways suspended service in parts of the country and replaced some routes with temporary bus routes, and cruise ships removed the ports of Ashdod and Haifa from their itineraries.

At least 44 nations denounced Hamas and explicitly condemned its conduct as terrorism, including a joint statement by the United States, the United Kingdom, France, Italy, and Germany. In contrast, Arab and Muslim countries including Qatar, Saudi Arabia, Kuwait, Syria, Iran and Iraq blamed Israel for the attack. The UAE, Bahrain, and China have all amended their initial declarations to expressly denounce the killing and abduction of Israeli civilians. According to a poll conducted by The Washington Institute for Near East Policy between 14 November and 6 December 2023, 95% of Saudis did not believe that Hamas had killed civilians in its attack on Israel.Over 680 legal experts and 128 human rights experts from Israel and around the world have signed an appeal for the immediate release of all hostages kidnapped by Hamas, and for the end of the "vicious and inhumane capture, violence, torture and other cruel, inhuman or degrading treatment of women and girls, children and infants." According to the appeal, "the abductees are defined according to international law as victims of enforced disappearance... [which makes these acts] blatant violations of international human rights law and humanitarian law, amounting to war crimes and crimes against humanity."The United Nations, particularly the United Nations Committee on the Elimination of Discrimination against Women (CEDAW), faced criticism for failing to condemn Hamas's actions against women and failing to voice disapproval of the mass rape of Israeli women and girls.After the attacks, the Shoah Foundation said it had gathered over 100 video testimonies of those who experienced the attacks to add them to the collection of "Holocaust survivor and witness testimony." Shoah Foundation founder Steven Spielberg said of the attacks, "I never imagined I would see such unspeakable barbarity against Jews in my lifetime" and that the Shoah Foundation project would ensure "that their stories would be recorded and shared in the effort to preserve history and to work toward a world without antisemitism or hate of any kind."

Israeli forces in Kfar Aza and Be'eri reported that they found bodies of victims mutilated. One IDF commander told an i24 News reporter that 40 babies had been killed, out of what one estimate described as at least 100 civilian victims. The story was later proved false; only two babies are known to have died as a result of the attack, one from a bullet, the other shortly after birth in hospital.

Other false reports of this type were spread by ZAKA volunteers acting as first responders. In one, a ZAKA volunteer said groups of children were found tied up and burned alive. Even so, actual atrocities were committed, from sexual assaults and rapes to mutilations; some victims were bound, and some victims' bodies were desecrated. At the music festival, there was said to be mass killing but less time for torture than at the kibbutz. About 70% of bodies were reported to have been shot in the back. Graeme Wood reported that the video footage retrieved from body cameras the attackers wore showed several victims who "in the beginning of the footage... are alive, [and] by the end they're dead. Sometimes, in fact frequently, after their death their bodies are still being desecrated." Other videos show attackers shooting at children, executing men in civilian clothing, throwing grenades into civilian shelters, and decapitations.First response personnel recovering the bodies reported being extremely distressed by the atrocities they witnessed, and said they placed the bodies of Hamas militants in body bags marked with an "X" and removed them with a bulldozer.Israeli security agencies released videos that the Times of Israel described as apparent interrogations of Hamas attackers, in which the subjects said they were ordered to kill, behead, cut off limbs and rape. A former chief rabbi of the Israeli army, part of the team identifying bodies, said there were many instances of rape and torture, and an Israeli reserve warrant officer said that forensic exams had discovered multiple cases of rape, though neither provided forensic evidence to support the claims. CNN has interviewed several Israelis who witnessed the aftermath of the attack, who reported visible signs of rape and excessive violence on the bodies of women and girls from several sites.A Ha'aretz investigation into the claims of mutilation and torture found that "Members of Hamas and Palestinian Islamic Jihad, as well as other Gazans who entered Israel, committed war crimes and crimes against humanity." Regarding "testimonies about Hamas' atrocities on October 7", Ha'aretz found that "Most are supported by extensive evidence, but a few have been proved untrue, providing ammunition to deniers of the historic massacre." Ha'aretz found several cases where Israeli search and rescue units, the army, and politicians disseminated inaccurate information. An Israeli army officer claimed that babies had been hung on clotheslines; later investigations showed that exactly one infant was killed, alongside her father, and that the reports of groups of children being slaughtered and mutilated were false. A total of five children under age six were killed, and another 14 between ages 12 and 15 were killed in rocket attacks from Gaza. Most of the children were killed alongside family members. Ha'aretz reported that "Hamas terrorists did desecrate corpses during the massacre, especially the bodies of soldiers. There were also beheadings and cases of dismemberment" but that "there is no evidence that children from several families were murdered together, rendering inaccurate Netanyahu's remark to U.S. President Joe Biden that Hamas terrorists 'took dozens of children, tied them up, burned them and executed them.'" ZAKA volunteers shared stories of atrocities, with one repeatedly describing 20 children having been bound and burned at a kibbutz; the same volunteer said a pregnant woman had her unborn baby cut from her womb and that he had found the woman next to two murdered children aged six and seven. But the list of dead does not correspond to the claims, and no children of that age were killed in the kibbutz; the kibbutz has denied that the story is related to the kibbutz. Sara Netanyahu, the Israeli prime minister's wife, sent U.S. first lady Jill Biden a letter claiming that a heavily pregnant woman was taken hostage to Gaza; the woman was identified as a Thai worker who had been taken hostage and later released. She was not pregnant and had not given birth.U.S. Secretary of State Antony Blinken described some of the evidence given by the same ZAKA volunteer: "a young boy and girl, 6 and 8 years old, and their parents around the breakfast table. The father's eye gouged out in front of his kids. The mother's breast cut off, the girl's foot amputated, the boy's fingers cut off before they were executed"; and "a baby, an infant, riddled with bullets. Soldiers beheaded. Young people burned alive. I could go on, but it's simply depravity in the worst imaginable way."

In the aftermath of the initial Hamas assault, witnesses from the Israeli soldiers, the Israeli Department Forces, and the first responder Israeli organization ZAKA said on French Israeli TV channel i24news that they had seen the bodies of beheaded infants at the site of the Kfar Aza massacre. During Antony Blinken's visit to Israel, he said he was shown photos of the massacre by Hamas of Israeli civilians and soldiers, and specifically that he saw beheaded IDF soldiers. U.S. President Biden separately said that he had seen photographic evidence of terrorists beheading children, but it was not the case; the White House subsequently clarified that Biden was alluding to news reports of beheadings, which have not contained or referred to photographic evidence. NBC News called reports of "40 beheaded babies" unverified allegations, adding that they appeared "to have originated from Israeli soldiers and people affiliated with the Israel Defense Force" and that "an Israeli official told CNN the government had not confirmed claims of the beheadings". The allegation mainly "stemmed from a viral Israeli news broadcast clip" and the main X / Twitter accounts propagating the claims were i24NEWS and Israel's official account, even though Israeli Defense spokesperson Doron Spielman told NBC News that he could not confirm i24NEWS's report. As of 12 October, CNN had extensively reviewed online media content to verify Hamas-related atrocities but found no evidence to support claims of decapitated children.An Israeli ZAKA volunteer reported on 14 October seeing children's bodies with severe injuries and burns. Some of the bodies appeared to have been decapitated, but the exact circumstances were not clear.According to The Jerusalem Post, which reprinted an article from the Israeli website Themedialine.org (whose founder, Felice Friedson, was praised by The Jerusalem Post and is a contributor to it), approximately 200 forensic pathologists and other experts—from Israel, Switzerland, New Zealand, the U.S. and elsewhere—reviewed evidence of the attack at the National Center of Forensic Medicine (Abu Kabir) in Tel Aviv. Chen Kugel, head of the center, said that many bodies, including those of babies, were without heads. When asked whether the bodies had been decapitated, Kugel answered yes. He added that it was difficult to determine whether the dead were decapitated before or after death, or whether their heads had been "cut off by knife or blown off by RPG".On 24 October, Israeli authorities screened bodycam footage of Hamas atrocities for journalists, including "an attempt to decapitate someone who appeared to be still alive using a garden hoe", as well as a still image of a decapitated IDF soldier.On 4 December, Haaretz reported that "unverified stories [had been] disseminated by Israeli search and rescue groups, army officers and even Sara Netanyahu". Haaretz journalists Nir Hasson and Liza Rozovsky related the chronology of the news items about "beheaded babies" and "hung babies" and concluded, "this story is false". They quoted Ishay Coen, a journalist for the ultra-Orthodox website Kikar Hashabbat, who admitted he made a mistake by unquestioningly accepting the IDF's claims. "Why would an army officer invent such a horrifying story?", Hashabbat asked, adding, "I was wrong." Haaretz also reported that some testimony came from reservist officers.

On 20 October, a forensic analysis was presented to the media at Israel's National Center of Forensic Medicine that claimed to show evidence of victims burned alive with bound hands. The analysis suggested that one CT scan of charred remains showed an adult bound to a child at the time of death. Many victims had soot in their trachea, indicating that they burned to death.

According to several international law and genocide studies experts, Hamas's assault amounted to genocide. Legal and genocide experts have condemned the attack, saying it represents a serious violation of international law. They argue that Hamas carried out these actions with the intent to destroy the Israeli national group. Some commentators point to Hamas's founding charter, which advocates for the destruction of Israel, contains antisemitic language, and, according to certain researchers, implies a call for the genocide of Jews. This has led to suggestions that the October 7 attacks were an effort to fulfill this agenda.