Israel has declared war on the Palestinian militant group Hamas after it carried out an unprecedented attack by air, sea and land on Saturday.
The large-scale surprise assault has left at least 900 dead in Israel, prompting a lethal volley of retaliatory Israeli airstrikes on Gaza that killed at least 765 people.
As they retreated into Gaza, the militants claimed to have taken at least 100 hostages with them and have threatened to kill them if airstrikes target Gaza without warning. Israel has pledged that Hamas will pay a heavy price and may now be preparing a ground incursion into Gaza.
Here's what we know so far.
What happened?
Militants from Gaza fired thousands of rockets towards Israeli towns on Saturday morning, before breaking through the heavily fortified border fence with Israel and sending militants deep into Israeli territory. There, Hamas gunmen killed hundreds of people, including civilians and soldiers, and took hostages, sometimes from their homes.
It took Israeli troops more than two days to take back control as fighting raged in the streets. On Monday, the Israel Defense Forces (IDF) said it had retaken control of all Israeli communities in Gaza's vicinity on its southern border after fighting with Hamas ended.
IDF spokesman Jonathan Conricus said Tuesday that the bodies of "about 1500" Hamas fighters have been recovered inside Israel since the attack on Saturday.
The attacks were unprecedented in tactic and scale as Israel has not faced its adversaries in street battles on its own territory since the 1948 Arab-Israeli war. It has also never faced a terror attack of this magnitude that has taken the lives of so many civilians. While Hamas has kidnapped Israelis before, it has never before taken dozens of hostages at once, including children and the elderly.
Hamas called the operation "Al-Aqsa Storm" and said that the assault was a response to what it described as Israeli attacks on women, the desecration of the al-Aqsa mosque in Jerusalem and the ongoing siege of Gaza.
How has Israel responded?
In response to the attack, Israel has declared war and launched "Operation Swords of Iron," striking suspected Hamas and Islamic Jihad targets in Gaza.
Hundreds had been killed in Gaza as of Monday, including dozens of children, the Palestinian Ministry of Health in Gaza said.
The IDF has urged civilians in Gaza to leave their residential areas immediately for their safety as Israeli military operations continue to target Hamas, and shut all crossings between Israel and Gaza, potentially setting the stage for a ground incursion into the enclave.
Israeli Minister of Defense Yoav Gallant said on Monday that he had ordered a "complete siege" on Gaza. "No electricity, no food, no fuel. Everything closed," he said, adding that no water will be delivered either.
How did the two sides get here?
Tensions between Israelis and the Palestinians have existed since before Israel's founding in 1948. Thousands of people on both sides have been killed and many more injured over decades.
Violence has been particularly acute this year. The number of Palestinians -- militants and civilians -- killed in the occupied West Bank by Israeli forces since the year began is the highest in nearly two decades. The same is true of Israelis and foreigners -- most of them civilians -- killed in Palestinian attacks.
Israel captured Gaza from Egypt in the 1967 war, then withdrew its troops and settlers in 2005. The territory, home to some 2 million Palestinians, fell under Hamas' control in 2007 after a brief civil war with Fatah, a rival Palestinian faction that is the backbone of the Palestinian Authority.
After Hamas seized control, Israel and Egypt imposed a strict siege on the territory, which is ongoing. Israel also maintains an air and naval blockade on Gaza. Human Rights Watch has called the territory an "open-air prison." More than half of its population lives in poverty and is food insecure, and nearly 80% of its population relies on humanitarian assistance.
Hamas and Israel have fought several wars. Before Saturday's operation, the last war between the two was in 2021, which lasted for 11 days and killed at least 250 people in Gaza and 13 in Israel.
Saturday's assault occurred 50 years almost to the day since the 1973 war, when Israel's Arab neighbors launched a surprise attack on Israel on Yom Kippur, the holiest day in the Jewish calendar, on October 6, 1973.
What is Hamas?
Hamas is an Islamist organization with a military wing that came into being in 1987, emerging out of the Muslim Brotherhood, a Sunni Islamist group that was founded in the late 1920s in Egypt.
The group, like most Palestinian factions and political parties, insists that Israel is an occupying power and that it is trying to liberate the Palestinian territories. It considers Israel an illegitimate state and has called for its downfall.
Unlike some other Palestinian factions, Hamas refuses to engage with Israel. In 1993, it opposed the Oslo Accords, a peace pact between Israel and the Palestine Liberation Organization (PLO) that saw the PLO give up armed resistance against Israel in return for promises of an independent Palestinian state alongside Israel. The Accords also established the Palestinian Authority (PA) in the Israeli-occupied West Bank.
Hamas presents itself as an alternative to the PA, which has recognized Israel and has engaged in multiple failed peace initiatives with it. The PA, whose credibility among Palestinians has suffered over the years, is today led by President Mahmoud Abbas.
Hamas has over the years claimed many attacks on Israel and has been designated as a terrorist organization by the United States, the European Union and Israel. Israel accuses its archenemy Iran of backing the group.
What happens next?
Israel is now on a war footing and has already started mobilizing troops for a potential ground operation in Gaza. It has said that it will exact a heavy price on Hamas for its attack and plans to retrieve Israeli hostages from the territory.
Israel has dealt with hostage situations before, but never at this scale. In the past, militants have mostly demanded the release of prisoners held in Israeli jails in exchange for captured Israelis. In 2011, Israel traded 1,027 Palestinians for Israeli soldier Gilad Shalit, and in 2004, it released more than two dozen Lebanese and Arab prisoners -- including two senior Hezbollah officials -- for Elhanan Tannenbaum, an Israeli businessman and army reserve colonel, as well as the bodies of three IDF soldiers. In 2008, Israel released five Palestinian prisoners, five Lebanese prisoners and returned the bodies of nearly 200 Arab fighters in exchange for the bodies of two Israeli soldiers.
Hamas says it has taken captive 100 or more hostages. Their presence in Gaza will undoubtedly complicate any Israeli military operation there.
The militant group's armed wing said Monday it would begin killing civilian hostages and broadcasting the act if Israel targets people in Gaza without warning.
It comes after the IDF said it planned to take control of the Gaza Strip. Its spokesperson, Lt. Col. Richard Hecht, said the aim is to "end the Gaza enclave" and "control the entire enclave."
When asked whether it had stopped the "knock on the roof," which is the Israeli military's warning for civilians before it bombs a building. Hecht responded that Hamas did not "knock on the roof."
"When they came in and threw grenades at our ambulances they did not knock on the roof. This is war. The scale is different," Hecht said.
Senior Hamas member Saleh al-Arouri told Al Jazeera Arabic on Saturday that Hamas is ready "for all options, including a war and an escalation on all levels."
"We are ready for the worst-case scenario, including a ground invasion, which will be the best for us to decide the ending of this battle," al-Arouri said.
Could this lead to a wider regional conflict?
Hamas' operation was carried out in a sophisticated and coordinated manner and would have taken a significant amount of planning. Speculation has been rife that the group may have received assistance from abroad, which, if proven, could raise the specter of a wider regional war.
Israel says Iran supports Hamas to the tune of some $100 million dollars a year. The US State Department in 2021 said that the group receives funding, weapons, and training from Iran, as well as some funds that are raised in Gulf Arab countries.
"Of course Iran is in the picture," one US official told CNN. "They've provided support for years to Hamas and Hezbollah."
A senior Biden administration official said on Saturday that it was too early to say whether Iran was directly involved in the attack, but that Washington will be tracking the matter "very closely."
Iranian President Ebrahim Raisi spoke to Hamas leader Ismail Haniyeh by phone on Sunday and later congratulated the Palestinian people for their "victory" over Israel. On Monday, however, Iran's mission to the United Nations said that the Islamic Republic was "not involved in Palestine's response," referring to the Hamas attack. "It is taken solely by Palestine itself," it said.
The US has meanwhile sent a carrier strike group to the eastern Mediterranean Sea, according to Defense Secretary Lloyd Austin. A US official told CNN that the US is also sending more fighter jets to the Middle East to deter any Iranian potential aggression or an expansion of the fighting beyond Israel's borders.
Israel may also face the threat of new fronts opening in the war. Of its immediate neighbors, it is only at peace with Jordan and Egypt, and is officially in a state of war with Lebanon and Syria. Israel has said it is ready in case there are attacks from those two countries.
The Lebanese militant group Hezbollah, which is backed by Iran, has praised Hamas' attack and said it is in contact with Palestinian militant groups "at home and abroad," its Al Manar channel said. On Sunday, the group claimed responsibility for targeting three Israeli sites in an area known as Shebaa Farms using missiles and artillery. The area is considered by Lebanon as Israeli-occupied. Israel responded by firing artillery.
On Monday, the IDF said it killed a "number of armed suspects" who infiltrated into Israel from Lebanon and that soldiers were searching the area. Lebanese Prime Minister-designate Najib Mikati said on Monday that his country doesn't want to be drawn into the conflict.