Hamas has launched an unprecedented strike on Israel. Here’s what you need to know.

The IDF has launched retaliatory strikes after scores of Israeli citizens are dead and hundreds wounded.

Hamas on Saturday launched a large, well-coordinated attack on Israel from the territory it controls in Gaza after months of simmering conflict between Israel and Palestinians in Jerusalem, the West Bank, and Gaza.

A group of Hamas militants entered Israel Saturday — an unprecedented breach of the security apparatus that controls Palestinian movement in and out of Israel — killing at least 40 Israelis and wounding at least 779, according to the New York Times. There have also been reports of Hamas fighters taking hostages back into Gaza and of gunfights in southern Israeli towns. Hamas military leader Mohammad Deif also claimed that the group had launched 5,000 rockets into Israel; the Israeli military put that number at 2,200 by late morning Saturday. Still, that number indicates the massive scale of this operation; in the whole of a 50-day war between Hamas and Israel in 2014, the group launched a total of 4,564 rockets and mortars into Israel.

“Our enemy will pay a price the type of which it has never known,” Israeli Prime Minister Benjamin Netanyahu said about the attacks. “We are in a war and we will win it.” Under Netanyahu’s leadership, Israel has become increasingly hostile to Palestinians and encouraged Israeli settlements in parts of the West Bank, another Palestinian enclave.

The Israeli Defense Force has already retaliated with air strikes against Gaza, which has suffered from blockades by Israel and Egypt for years and has been described as an “open-air prison.” Meanwhile Iran and Hezbollah, the Iran-backed Shia militant group based in southern Lebanon, have praised the attacks.

The attacks came on the final day of Sukkot, a Jewish celebration of the harvest, as well as on the 50th anniversary of the start of the 1973 Yom Kippur war, in which Egypt and Syria attacked Israel, threatening the existence of the young nation.