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CNN  — 

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 more than 1200 dead in Israel, according to public broadcaster Kan. It prompted a lethal volley of retaliatory Israeli airstrikes on Gaza that has killed at least 900 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.