What is Hamas? The militant group behind surprise attack on Israel has ruled Gaza for years

Hamas, the Islamic militant group that controls the Palestinian territory of Gaza, on Saturday launched an unexpectedly complex and deadly attack on Israel that officials said included waves of rocket launches and hundreds of invaders.

The assault has so far left more than 900 dead in the country, according to Israeli health officials, and is expected to engender a fierce response from the Israeli military, which has been involved in clashes with Palestinian militants in the occupied West Bank throughout the year.

Already, more than 900 people have been killed in Gaza, according to the Palestinian Health Authority.

More than 7,000 people have been injured total.

Here's what to know about Hamas.

What is Hamas

Hamas has a membership of between 20,000 and 25,000, according to the U.S. government, and was founded in 1987 as an offshoot of the Egyptian Muslim Brotherhood. Its name is an acronym for Harakat al-Muqawama al-Islamiya, or Islamic resistance movement.

Hamas rejects Israel's right to exist and is dedicated instead to the creation of a Palestinian country in the region.

The group has both sociopolitical and military functions and receives crucial support and weapons from Iran. It is one of several organizations vying for influence over Palestinians in the decadeslong conflict over their future since the modern state of Israel was created in what had been Mandatory Palestine.

Hamas is labeled a terrorist organization by the U.S., the European Union, the U.K. and others. A resolution to label Hamas a terrorist group failed to pass in the U.N. in 2018.

Hamas de facto seized Gaza -- also known as the Gaza Strip, one of the two major Palestinian territories -- following election wins there in 2006 and a violent conflict with the competing Fatah political group in 2007. Since then, Fatah has continued to govern in the West Bank territory to the northeast.

Past polling has shown Hamas has variable levels of support among Palestinians, though it sometimes has the edge over Fatah -- with one pollster previously telling the Associated Press that Hamas is embraced during conflict.

Hamas has repeatedly clashed with Israel since taking over the exclave of Gaza, which is home to some 2 million people and is about half the size of New York City.

Gaza is under a strict blockade by Israel and Egypt that sharply restricts the movement of people and goods into and out of the area. In the wake of the Hamas attack this weekend, Israeli defense officials said the flow of all food and power into Gaza was being cut off in preparation for a "total siege."

Some of the most serious fighting between Hamas and Israel took place in 2008, when Israel sent troops into the Gaza that resulted in fierce urban fighting in an attempt to quell ongoing rocket launches.

This year, before this weekend's attack, the group has sporadically fired rockets into Israel, coinciding with rising tensions between Israel and Palestinians in the West Bank and expanded access granted to Jews in the area surrounding Jerusalem's al-Aqsa Mosque, a holy site for Muslims that also sits on a holy site for Jews.

Hamas also has ties to other armed groups in Gaza, including Palestinian Islamic Jihad, which at times conducts its own attacks and is thought to be more extreme.

Beyond its militancy, Hamas administers government and social services in Gaza.

Palestinian Authority President Mahmoud Abbas, Fatah's leader, has said his West Bank-based group and Hamas had papered over their differences in 2012, though the two are still considered rivals.

A third group, the Palestine Liberation Organization, also chaired by Abbas, remains the U.N.-recognized "representative of the Palestinian people."

Palestine is recognized by the U.N. as a "non-member observer state," though Palestinians largely live in the two territories adjacent to Israel pending some future resolution to their statehood.

The U.S., while not officially recognizing Palestinian statehood, says it works with Palestinian Authority in pursuit of "a secure, free, democratic, and stable Palestinian society and governance" in what the U.S. has long maintained must be a negotiated deal in tandem with securing Israel's own future.

What's behind the Hamas-Israel conflict

Hamas officials have cited escalating tensions around the al-Aqsa Mosque as a chief reason for the most recent attack.

Israel's government, now led by a coalition that includes hard-line nationalist parties, has in recent years granted increasingly expansive access for Jews to the area around the site in East Jerusalem, which Israel annexed decades ago though its possession isn't recognized internationally. (Neighboring Jordan officially administers the area, due to its religious importance, though Israel exercises security control.)

Tensions were already high this year over further Israeli settlement expansion in the West Bank -- which the U.N. considers illegal and which Palestinians and the U.S. have said is counter-productive to a possible two-state agreement -- and repeated clashes between the Israeli military and militants in the territory, particularly in the Jenin refugee camp.

What's next?

The death toll from the Hamas attack is expected to rise as Israel continues to repel Hamas fighters in the country while launching retaliatory operations, including numerous of its own strikes on Gaza.

President Joe Biden has signed off on sending "additional assistance" to Israel, "with more to follow over the coming days."

In the past, Israel has waged wide-scale air assaults on the Gaza Strip. Israel has claimed past operations have targeted Hamas' military infrastructure, but the assaults have come under international criticism for causing widespread civilian casualties as well.

Israel has appeared reluctant to launch ground invasions of Gaza after its troops were killed in the 2008 fighting there. However, in the wake of this weekend's attack, Israeli Prime Minister Benjamin Netanyahu promised to "destroy Hamas."

According to Israel, as of Sunday at least 100 soldiers and civilians were being held hostage -- captives that Hamas told Al Jazeera it intends to use as part of a swap.