What is Hamas, and why did it attack Israel now?

Israel declared war against Hamas on Sunday following a surprise attack by the Palestinian militant group based in Gaza that included the taking of military and civilian hostages. Israeli security forces, caught off-guard, have pounded Gaza with retaliatory strikes, and U.S. officials said they expect Israel to soon launch a ground incursion into Gaza as violence escalates in the conflict-ridden region.

More:Israel declares war, bombards Gaza and battles to dislodge Hamas fighters after surprise attack

Since winning an election in the Gaza Strip in 2006, Hamas has repeatedly attacked Israel with rockets and mortars, emerging as a defiant adversary. Israel has retaliated with its superior firepower and a punishing blockade, restricting imports and movement of civilians in a strategy of collective punishment. The blockade and recurring Israeli strikes have contributed to Gaza's poor infrastructure and living conditions. Israel declared a full siege of the enclave on Monday, with Defense Minister Yoav Gallant promising "no electricity, no food, no fuel" and calling Hamas "savages."

Here's what to know about Hamas and the latest violence.

What is Hamas?

Hamas, or the Islamic Resistance Movement, is a militant group that governs the Gaza Strip, a 25-mile-long, densely populated enclave of more than 2.1 million people. Hamas emerged as an offshoot of the Muslim Brotherhood during the first Palestinian intifada, or uprising, in 1987 against the Israeli occupation of Gaza and the West Bank. It was founded by Sheikh Ahmed Yassin, a Palestinian cleric. Its military wing, the Izzedine al-Qassam Brigades, was established around 1991.

It is committed to armed resistance against Israel and aims for the creation of a Palestinian state with Jerusalem as its capital, along the borders before the Six-Day War in June 1967, during which Israel captured and occupied the West Bank, Gaza, the Sinai Peninsula and the Golan Heights. Hundreds of thousands of Palestinians were displaced as a result.

In October 1997, the United States designated Hamas a terrorist organization. The group has used explosives and rockets along with suicide bombings and kidnappings to target Israel.

Hamas won the elections in Gaza in 2006, defeating Fatah, the main Palestinian party that still controls the West Bank.

Hamas's top political leader, Ismail Haniyeh, is based in Qatar, one of the countries that funds the group besides Iran.

Israel has targeted Hamas leaders over the years. In 1997, Khaled Meshal, a top official, survived an assassination attempt by Mossad, Israel's secret service agency, which poisoned him in Amman, Jordan. Meshal was saved after Jordan detained the Israeli agents and President Bill Clinton pressed Israel to hand over the antidote.

Israel assassinated Yassin and another founding member, Abdel Aziz Rantisi, in 2004 and killed Hamas military chief Ahmed Jabari in November 2012.

Why did Hamas attack Israel now?

The coordinated attack by Hamas caught Israel by surprise but comes after months of worsening tensions over violence at al-Aqsa Mosque - a deeply revered Muslim holy site in the heart of Jerusalem - as well as the punishing blockade and occupation of Palestinians. Once-fringe Jewish supremacists and settler leaders have been given key positions in Prime Minister Benjamin Netanyahu's hard-right government, further inflaming tensions.

Palestinian anger also reached a boiling point in May 2021 over the evictions of families from a Palestinian neighborhood in Jerusalem, leading to clashes between protesters and Israeli forces that prompted Hamas to launch rockets at Israeli cities.

In the months leading up to the surprise attack, there had been increasing numbers of clashes between Israeli forces and Palestinians, particularly in the West Bank. Between January and September this year, 227 Palestinians had been killed by Israeli forces or settlers - more than the number in all of 2022, according to the United Nations. Israeli fatalities, before the latest violence, totaled at least 29.

During the holy month of Ramadan, in April, Israeli forces stormed al-Aqsa Mosque in Jerusalem and used force on worshipers including women and the elderly. The third holiest site in Islam, al-Aqsa is considered the place from which the prophet Muhammad ascended to heaven.

It is also located on the holiest site in Judaism, known to Jews as the Temple Mount. For both Jews and Muslims, it is the site where Abraham offered to sacrifice his son.

A five-day conflict broke out in May between Israel and Islamic Jihad, another armed Palestinian faction, killing at least 33 in Gaza and two in Israel.

Over the summer, there was a series of clashes between Palestinian militants in the West Bank and Israeli forces or Jewish settlers. In June, four Israelis were killed after two Hamas gunmen opened fire at a hummus restaurant outside an Israeli settlement.

Days later, Israel carried out its most expansive military operation in two decades in the West Bank when it stormed the city of Jenin with about 1,000 soldiers backed by drone strikes. Calling it a "counterterrorism" effort, the operation focused on the impoverished Jenin refugee camp, known to be a hub of armed factions, many with links to Hamas and Islamic Jihad.

Tensions in Gaza nearly spilled over in September after Israeli agents found explosives in a shipment of jeans and ended all exports from the enclave. In response, Hamas held field exercises, including practice rocket launches, and allowed Palestinians to protest at the border fence separating Israel and Gaza.

How is Hezbollah involved?

On Sunday, Hezbollah, Lebanon's Islamist militant group and a major political party, said it attacked Israeli targets in Shebaa Farms, a disputed area on the border, "in solidarity" with Hamas, prompting a retaliation from Israel. While the group did not say it was formally joining the fight, its message of support raised the likelihood of a wider regional conflict.

Hezbollah has long been seen as a more dangerous opponent to Israel than Hamas, with more sophisticated weaponry and closer ties to Iran.

"Our hearts are with you. Our minds are with you. Our souls are with you. Our history and guns and our rockets are with you," top Hezbollah official Hashem Safieddine said at a rally in Beirut on Sunday, in reference to Hamas.

While Palestinian factions in recent years have used Lebanese territory to attack Israel, Hezbollah's behavior had indicated it aimed to avoid escalation, and attacks were usually limited, often targeting uninhabited lands. But Sunday's moves of trading fire and public comments suggest it does not intend to remain on the sidelines.

Israel requested cooperation with the United States on sharing intelligence related to southern Lebanon, officials told The Washington Post.