What is Hezbollah? Lebanon's militant group has long been one of Israel's biggest foes

Israel and Hezbollah's long-simmering border conflict has intensified in recent days, piquing fears of escalation.

Israel and Hezbollah have exchanged hundreds of cross-border strikes in recent days, following last week's widespread, deadly attacks using exploding pagers and walkie-talkies targeting Hezbollah members in Lebanon and Syria. Israel was behind the covert operation, sources told ABC News, though the country has not publicly taken responsibility for the operation.

Israel and Hezbollah, an Iran-backed terror group, have been engaged in mostly tit-for-tat cross-border fire for the better part of a year, following Hamas' surprise attack on Israel from Gaza on Oct. 7, 2023. The next day, as Israel initiated its retaliatory war on Hamas, Hezbollah began sending rockets over its border to Israel, vowing solidarity with Hamas.

Here's what to know about Hezbollah.

What is Hezbollah?

Hezbollah -- which means "party of God" or "party of Allah" -- is an Iran-backed, Shiite Muslim political party and militant group based in Lebanon. It was founded in the 1980s amid the 15-year Lebanese Civil War and has been led by its Secretary-General Hassan Nasrallah since 1992. Today, it controls much of the Shiite-majority areas of Lebanon, including parts of the capital, Beirut.

Hezbollah has been designated a terrorist organization by multiple countries, including the United States and Israel.

The group has clashed for decades with Israel, which is considered Hezbollah's "main enemy," going back to Israel's occupation of southern Lebanon in 1978, according to the Council on Foreign Relations, a New York City-based independent think tank.

"Even after Israel officially withdrew from southern Lebanon in 2000, it continued to clash with Hezbollah," including in a monthlong war in 2006, according to the Council on Foreign Relations.

Hezbollah has been blamed for multiple terrorist attacks on Jewish and Israeli targets abroad. Its reach isn't limited to Lebanon, with "evidence of Hezbollah operations in Africa, the Americas and Asia," according to the Council on Foreign Relations.

The group has also been involved in the Syrian civil war, joining Iran and Russia in support of the Syrian government.

Hezbollah was also behind the 1983 U.S. Marine Corps barracks bombing in Beirut, which marks the single deadliest day for the Marine Corps since the Battle of Iwo Jima in World War II. On Oct. 23, 1983, 241 U.S. military personnel were killed when a suicide bomber detonated a truck bomb into the Marine Corps barracks. A second suicide bomber killed 58 French paratroopers. Six innocent Lebanese civilians were also killed.

As a political party, Hezbollah and its allies lost their majority in Lebanon's 128-member Parliament in its 2022 elections. Though "it remains the dominant party of the Shiite community and an extremely powerful armed group that does not hesitate to use force inside, and outside, the country to pursue its or Iran's goals," the Middle East Institute, a Washington, D.C.-based nonpartisan think tank, said at the time.

What is the difference between Hamas and Hezbollah?

Hamas and Hezbollah are united in rejecting Israel's right to exist and are considered terrorist groups by the U.S., Israel and other countries.

They are both Islamic militant groups, though Hamas belongs to the Sunni branch and Hezbollah the Shia branch.

Hamas controls the Palestinian territory of Gaza, located on Israel's southwestern border, while Hezbollah is based in Lebanon, on Israel's northern border. Lebanon has been without a president since 2022, and its government has remained weak due to its fragmented political systems.

"By 2023, Hezbollah was often described as a 'state within a state' because it was part of the Lebanese parliament and government while also operating its own political, military and social services network with a great degree of impunity," the Wilson Center said.

Iran has long been known to provide support, training and weapons to both armed resistance groups, which are considered to be the largest in a network of Tehran proxy groups on Israel's borders.

"Iran seems to be the head of the 'octopus of terror.' One arm Hezbollah, another Hamas," Naftali Bennett, the former prime minister of Israel, said during a briefing organized by Jewish Federations of North America in the wake of Hamas' surprise attack in Israel on Oct. 7, 2023.

Why are Israel and Hezbollah fighting?

Fighting between the Israel Defense Forces and Hezbollah has been constant since Oct. 8, 2023, when Israel invaded the Gaza Strip targeting Hamas over the terrorist group's surprise attack on Oct. 7. Hezbollah began attacks on Israel in protest, saying it will continue its attacks until Israeli forces withdraw from the Palestinian territory.

Tens of thousands of Israelis fled border regions under Hezbollah fire since the fighting began. Their return is a priority for Israel.

"We will take whatever action is necessary to restore security and to bring our people safe back to their homes," Israeli Prime Minister Benjamin Netanyahu said on Sunday.

As the Israeli military ramped up its airstrikes in Lebanon this week, including striking more than 1,600 Hezbollah targets in southern Lebanon on Monday, Netanyahu said that Israel was changing "the security balance, the balance of power in the North."

"For those who have not yet understood, I want to clarify Israel's policy -- we do not wait for a threat, we anticipate it," Netanyahu said in a statement Monday. "Everywhere, in every arena, at any time. We eliminate senior officials, eliminate terrorists, eliminate missiles."

Israeli leaders are also demanding that Hezbollah withdraw beyond the Litani River -- some 18 miles north of the Israeli border -- as stipulated in a 2006 United Nations Security Council resolution that sought to end the last major cross-border war.

"If the world does not withdraw Hezbollah north of Litani in accordance with Resolution 1701 -- Israel will do so," Israeli Foreign Minister Israel Katz said on Sunday.

The conflict intensified last week with the detonation of Hezbollah communication devices in Lebanon and Syria, which Nasrallah blamed on Israel and described as an "unprecedented blow" for the group.

Two consecutive days of electronic device explosions -- which the Lebanese health minister said killed more than two dozen people and wounded thousands -- were followed by the killing of Hezbollah operations chief Ibrahim Aqil and 14 other members in a Beirut airstrike.

Hezbollah leaders said they would continue their operations despite last week's setbacks.

Deputy Secretary General Naim Qasem, Hezbollah's second-in-command, spoke at Aqil's funeral in Beirut on Sunday, telling hundreds of mourners that the conflict has now entered "a new phase" that he called an "open-ended battle of reckoning."

"Threats won't stop us, and we don't fear the most dangerous possibilities," he continued. "We are ready to face all military scenarios."

The U.S. is continuing to urge Israel to avoid an "all-out war" with Lebanon as tensions between the two countries remain high, National Security Council spokesperson John Kirby said Tuesday in an interview on "Good Morning America."

"I think we don't believe it's in Israel's interest for this to escalate, for there to be an all-out war there on the north on that blue line between Israel and Lebanon. If the goal is to get families back to their homes, we think there's a better way to do that than an all-out conflict," Kirby said.

ABC News' David Brennan and Julia Reinstein contributed to this report.