Over the past month, Hezbollah has frequently targeted Israeli armour and military positions from across the border in Lebanon.
Hezbollah is a Shia Muslim organisation which is politically influential and in control of the most powerful armed force in Lebanon.
It was established in the early 1980s by the region's most dominant Shia power, Iran, to oppose Israel. At the time, Israel's forces had occupied southern Lebanon, during the country's civil war.
Hezbollah has participated in national elections since 1992 and has become a major political presence.
Its armed wing has carried out deadly attacks on Israeli and US forces in Lebanon. When Israel withdrew from Lebanon in 2000, Hezbollah took credit for pushing them out.
Since then, Hezbollah has maintained thousands of fighters and a huge missile arsenal in southern Lebanon. It continues to oppose Israel's presence in disputed border areas.
It is designated a terrorist organisation by Western states, Israel, Gulf Arab countries and the Arab League.
In 2006, a full-blown war broke out between Hezbollah and Israel, triggered when Hezbollah carried out a deadly cross-border raid.
Israeli troops invaded southern Lebanon to try to eliminate the threat from Hezbollah. However, it survived and has since increased its number of fighters and obtained new and better weapons.
Sheikh Hassan Nasrallah is a Shia cleric who has led Hezbollah since 1992.
He played a key role in turning it into a political, as a well as a military, force.
He has close links with Iran and its Supreme Leader, Ayatollah Ali Khamenei.
These date from 1981, when Iran's first Supreme Leader, Ayatollah Ruhollah Khomeini, appointed him his personal representative in Lebanon.
Nasrallah has not appeared in public for years, purportedly for fear of being assassinated by Israel.
However, he remains revered by Hezbollah, and delivers televised speeches every week.
More on Israel-Gaza war
Hezbollah is one of the most heavily-armed, non-state military forces in the world. It is funded and equipped by Iran.
Sheikh Hassan Nasrallah has claimed that it has 100,000 fighters, although independent estimates vary between 20,000 and 50,000.
Many are well trained and battle hardened, andfought in the Syrian civil war.
It has an estimated 130,000 rockets and missiles,according to the Center for Strategic and International Studies think tank.
Most of its arsenal is made up of small, unguided surface-to-surface artillery rockets.
But it also thought to have anti-aircraft and anti-ship missiles, as well as guided missiles capable of striking deep inside Israel.
This is much more sophisticated than what Hamas, in the Gaza Strip, has at its disposal.
Since the Hamas attack on 7 October, Hezbollah has been attacking military positions in northern Israel.
It has launched rockets at Israeli positions, fired anti-tank missiles at armoured vehicles and attacked military targets with explosive drones.
The Israel Defense Forces (IDF) have retaliated, using air strikes, tank and artillery fire against Hezbollah positions in Lebanon.
Israeli soldiers and Hezbollah fighters have been killed, and Israel has evacuated thousands of civilians from the area.
IDF spokesman Daniel Hagari said its forces were on "very high alert" along Israel's northern border.
The Israeli Prime Minister, Benjamin Netanyahu, has promised a response of "unimaginable" magnitude if Hezbollah opens a second front in the conflict.
Hassan Nasrallah, the leader of the Islamist group Hezbollah, has praised the Hamas attacks on Israel in his first speech about the war.