The Palestinian militant group Hamas has launched an unprecedented attack on Israel, with its fighters entering communities near the Gaza Strip, killing residents and taking hostages.
Here's what you need to know about the people and places involved - and the essential context to understand this story.
What is Hamas?
Hamas is a Palestinian Islamist militant group which rules the Gaza Strip. Hamas is sworn to Israel's destruction and has fought several wars with Israel since it took power in Gaza in 2007.
In between those wars, it has fired or allowed other groups to fire thousands of rockets at Israel, and carried out other deadly attacks. Israel has also repeatedly attacked Hamas with air strikes, and, together with Egypt, has blockaded the Gaza Strip since 2007 in what it says is for its security.
Hamas as a whole, or in some cases its military wing, is designated a terrorist group by Israel, the United States, the European Union and the UK, as well as other powers. Hamas is backed by Iran which funds it and provides weapons and training.
What is the Gaza Strip?
The Gaza Strip is a 41km (25-mile) long and 10km-wide territory between Israel, Egypt and the Mediterranean Sea. It is home to about 2.3 million people and has one of the highest population densities in the world.
Israel controls the air space over Gaza and its shoreline and restricts who and what goods are allowed in and out through its border crossings. Similarly, Egypt controls who passes in and out through its border with Gaza.
About 80% of the population of Gaza depends on international aid, according to the UN, and about one million people rely on daily food aid.
What is Palestine and what have these events got to do with it?
The West Bank and Gaza, which are known as the Palestinian territories, as well as East Jerusalem and Israel all formed part of land known as Palestine since Roman times.
These were also the lands of Jewish kingdoms in the Bible, and are seen by Jews as their ancient homeland.
Israel was declared a state in 1948, though the land is still referred to as Palestine by those who do not recognise Israel's right to exist. Palestinians also use the name Palestine as an umbrella term for the West Bank, Gaza and East Jerusalem.
Why are Israel and Hamas fighting?
There is constant tension between Israel and Hamas, but the attack by the militants on Saturday came without warning. Hamas fired thousands of rockets at Israel while dozens of fighters breached the border and invaded Israeli communities, killing dozens of civilians and taking others captive.
Israel launched immediate air strikes, saying it was targeting militant sites in Gaza.
How unprecedented is this attack?
As our International Editor Jeremy Bowen writes, this is the most ambitious operation Hamas has ever launched from Gaza and the most serious cross-border attack Israel has faced in more than a generation.
Militants breached the wire that separates Gaza from Israel in multiple places.
The unprecedented attack came a day after the 50th anniversary of the surprise attack by Egypt and Syria in 1973 that started a major Middle East war. The significance of the date will not have been lost on the Hamas leadership.
Is this a major Israeli intelligence failure?
Yes, says our security correspondent Frank Gardner. With the combined efforts of Shin Bet, Israeli domestic intelligence, Mossad, its external spy agency and all the assets of the Israel Defense Forces, he says it is frankly astounding that nobody saw this coming or failed to act on it if they did have a warning.
Israel has arguably the most extensive and well-funded intelligence services in the Middle East, with informants and agents inside Palestinian militant groups, as well as in Lebanon, Syria and elsewhere.
On the ground, along the tense border fence between Gaza and Israel there are cameras, ground-motion sensors and regular army patrols.
The barbed-wire topped fence is supposed to have been a "smart barrier" to prevent exactly the sort of infiltration that has taken place in this attack. Yet the militants of Hamas simply bulldozed their way through it, cut holes in the wire or entered Israel from the sea and by paraglider.
What could happen next?
Hamas militant commander Mohammed Deif has called on Palestinians and other Arabs to join the militants' operation to "sweep away the [Israeli] occupation".
A big question now is whether Palestinians in the occupied West Bank and East Jerusalem or elsewhere in the region will heed his call, says our Jerusalem correspondent Yolande Knell.
Israel undoubtedly sees the potential for a war that could open up on multiple fronts. A worst-case scenario is that it could draw in the powerful Lebanese militant group, Hezbollah. On Sunday morning, Hezbollah launched a number of missiles and shells at northern Israel, without causing casualties.
The Israeli military has ordered a massive reinforcement of troops. As well as its intense air raids on Gaza, it has indicated that it is planning a ground operation there.
