Lebanon's Hezbollah has been exchanging fire with Israeli forces across the border since its Palestinian ally Hamas in Gaza and Israel went to war on Oct. 7.
The violence on the frontier between Israel and Lebanon is the deadliest escalation since Hezbollah and the Israeli military fought a major war in 2006.
WHAT ARE THE ORIGINS OF HEZBOLLAH?
Iran's Revolutionary Guards founded Hezbollah in 1982, in the middle of Lebanon's 1975-90 civil war. It was part of Iran's effort to exports its 1979 Islamic Revolution around the region and fight Israeli forces after their 1982 invasion of Lebanon.
Sharing Tehran's Shi'ite Islamist ideology, Hezbollah recruited Lebanese Shi'ite Muslims.
The group has risen from a shadowy faction to a heavily armed force with major sway over the Lebanese state. The United States, some Western governments and others deem it a terrorist organisation.
HOW POWERFUL IS HEZBOLLAH'S MILITARY?
While other groups disarmed after Lebanon's civil war, Hezbollah kept its weapons to fight Israeli forces that were occupying the predominantly Shi'ite Muslim south of the country. Years of guerrilla warfare led Israel to withdraw in 2000.
Hezbollah demonstrated its military advances in 2006 during a five-week war with Israel, which erupted after it crossed into Israel, kidnapping two soldiers and killing others.
Hezbollah fired thousands of rockets into Israel during the conflict, in which 1,200 people were killed in Lebanon, mostly civilians, and 158 Israelis were killed, most of them soldiers.
Hezbollah's military power grew after it deployed into Syria, another of Iran's allies in the region, to help President Bashar al-Assad fight mostly Sunni Muslim rebels.
Hezbollah boasts weapons including precision rockets and drones, and says it can hit all parts of Israel. In 2021, Hezbollah leader Sayyed Hassan Nasrallah said the group had 100,000 fighters.
Iran gives Hezbollah weapons and money. The United States estimates Iran has allocated it hundreds of millions of dollars annually in recent years.
WHAT'S HEZBOLLAH'S ROLE IN THE ISRAEL-HAMAS CONFLICT SO FAR?
Hezbollah has deep ties to Hamas, which controls Gaza, and Islamic Jihad, another Palestinian faction backed by Iran.
Hezbollah said it was in "direct contact with the leadership of the Palestinian resistance" on Oct. 7, the day Hamas militants carried out an assault on Israel, an attack that prompted Israeli airstrikes and a ground assault on Gaza.
Since Oct. 7, Hezbollah has been involved in increasingly heavy exchanges of cross-border fire with Israel. Hamas and Islamic Jihad, which both have a presence in Lebanon, have mounted attacks on Israel from Lebanon for the first time, including an Oct. 10 cross-border infiltration into Israel by Islamic Jihad.
Israeli Prime Minister Benjamin Netanyahu said on Oct. 22 that, if Hezbollah opened a war front with Israel, it would lead to counter-strikes of "unimaginable" magnitude on Lebanon.
WHAT REGIONAL SWAY DOES HEZBOLLAH HAVE?
Hezbollah has been a source of inspiration and support for other Iranian-backed groups across the Middle East. It has trained armed groups in Iraq and taken part in fighting there.
Saudi Arabia says Hezbollah has also fought in support of the Iran-allied Houthis in Yemen. Hezbollah denies this.
WHAT IS HEZBOLLAH'S ROLE IN LEBANON?
Hezbollah's influence is underpinned by its sophisticated arsenal and the support of many Lebanese Shi'ites who say the group defends Lebanon from Israel.
Lebanese parties opposed to Hezbollah say the group has undermined the state and accuse it of unilaterally dragging Lebanon into armed conflicts.
Hezbollah has ministers in government and lawmakers in parliament.
It entered Lebanese politics more prominently in 2005 after Syria withdrew forces from Lebanon following the killing of former prime minister Rafik al-Hariri, who symbolised Saudi influence in the country.
A U.N.-backed court convicted three Hezbollah members in absentia over the assassination. Hezbollah denies any role, describing the court as a tool of its enemies.
In 2008, a power struggle between Hezbollah and its Lebanese political adversaries, who had the backing of the West and Saudi Arabia, spiralled into a brief conflict. Hezbollah fighters took over parts of Beirut after the government vowed to take action against the group's military communications network.
In 2016, Hezbollah-allied Christian politician Michel Aoun became president - in Lebanon's sectarian political system the presidency is held by a Maronite Christian.
Two years later, Hezbollah and its allies won a parliamentary majority. This majority was lost in 2022, but the group continued to exercise major political sway.
The group campaigned against a judge investigating the 2020 Beirut port explosion, which devastated swathes of the capital, after he sought to question Hezbollah's allies. The standoff prompted deadly clashes in Beirut in 2021.
HAS HEZBOLLAH BEEN ACCUSED OF ATTACKS ON WESTERN TARGETS?
Lebanese security officials and Western intelligence have said groups linked to Hezbollah carried out suicide attacks on Western embassies and targets, and kidnapped Westerners in the 1980s. One group, Islamic Jihad, which is not related to the Palestinian organisation, was thought to be led by Imad Moughniyah, a top Hezbollah commander who was killed in a car bomb in Syria in 2008.
The United States holds Hezbollah responsible for a suicide bombing that destroyed the U.S. Marine headquarters in Beirut in 1983, killing 241 servicemen, and a suicide attack the same year on the U.S. embassy. A suicide bombing also hit a French barracks in Beirut in 1983, killing 58 French paratroopers.
Referring to those attacks and hostage-taking, Hezbollah leader Nasrallah said in a 2022 interview that they were carried out by small groups not linked to Hezbollah.
WHAT DO WESTERN GOVERNMENTS OR OTHERS SAY ABOUT THE GROUP?
Western countries including the United States designate Hezbollah a terrorist organisation. So do U.S.-allied Gulf Arab states including Saudi Arabia.
The European Union classifies Hezbollah's military wing as a terrorist group, but not its political wing.
Argentina blames Hezbollah and Iran for the 1994 bombing of a Jewish community centre in Buenos Aires in which 85 people were killed, and for a 1992 attack on the Israeli embassy in Buenos Aires that killed 29 people. Iran and Hezbollah both deny responsibility.
