Hezbollah militants in Lebanon have been exchanging fire with the Israeli military for nearly a year, but now the long-simmering conflict is approaching a boiling point.

Israel bombed more than 1,000 sites in southern and eastern Lebanon on Monday. Lebanon’s health minister said the strikes killed 558 people, with another 1,800 injured, making it the country’s deadliest day since a civil war that ended in 1990. The airstrikes continued on Tuesday.

Lebanon was already reeling from a string of Israeli attacks last week. But Hezbollah has not backed down. The armed group has launched more than 100 missiles, rockets and drones into Israel since Sunday, most of which were intercepted. There were no deaths or serious injuries reported in Israel.

World leaders, meeting at the United Nations General Assembly in New York this week, fear the conflict could quickly escalate even further and draw in bigger, more powerful players, like Hezbollah’s patron, Iran.

Here’s what to know.

What is Hezbollah?

Hezbollah is a Shiite Muslim militant group formed in the 1980s during the chaos of Lebanon’s long civil war. It was created, with Iranian guidance, to fight the Israeli occupation of southern Lebanon, which ended in 2000. The name Hezbollah (pronounced hez-bo-llah) is Arabic for “Party of God.”

Hezbollah has grown into a powerful political and fighting force in Lebanon, and has expanded its operations into Syria, Iraq, Yemen and elsewhere in the Middle East. It has an extensive infrastructure in Lebanon to support its mission, including offices dedicated to social services, communications and internal security.