Lebanon's powerful Shiite Muslim party Hezbollah, whose military wing was blacklisted by the European Union on Monday, is a key client of Iran and an ardent ally of Syria's regime.
Hezbollah (The Party of God) was founded in 1982 in response to Israel's invasion of Lebanon. It spearheaded an anti-Israeli guerrilla campaign and was the key actor in forcing the Jewish state to finally withdraw in 2000.
That success, coupled with the movement's extensive network of social services, has gained it enormous support among Lebanon's Shiites, who represent about a third of the country's population and the largest single community.
Widely considered more powerful than Lebanon's army, Hezbollah never disarmed at the end of the 1975-1990 civil war on the grounds it was necessary to protect the country against Israeli attacks.
Hezbollah wields great influence on Lebanese politics.
It, and its allies, have ministers in the government and control several seats in parliament -- which allows them to veto any policy or legislation not to their liking.
Though the EU sought to differentiate between Hezbollah's political and military wings, the reality is the movement makes no such distinction because it defines itself as a party of the anti-Israeli resistance and all its activities as linked to the cause.
Hezbollah has a formidable security and intelligence apparatus and is known abroad primarily for its military prowess.
Although Hezbollah initially sought to impose an Islamic way of life in multi-confessional Lebanon, it has since toned down its rhetoric.
The party drew its inspiration from the 1979 Islamic revolution in Shiite Iran, and has long been the recipient of financial and military backing from Tehran.
Much of the weaponry has been funnelled to it by the regime of Syrian President Bashar al-Assad, including thousands of missiles.
Those weapons played a key role in a second event that raised Hezbollah's popularity in Lebanon and the wider Arab world -- the 2006 war against Israel.
Hezbollah guerrillas carried out a daring cross-border raid, capturing two soldiers. Israel retaliated with a fierce air and land assault on Lebanon that devastated much of the Shiite-majority south and parts of Beirut itself.
But rocked by hundreds of Hezbollah's missiles and stymied by a fierce resistance on the ground, Israel never managed to vanquish the movement, and the party emerged even stronger.
Intelligence officials now estimate Hezbollah has amassed an arsenal of more than 40,000 short- and long-range rockets the party claims can reach deep inside Israel, as well as anti-aircraft guns and other sophisticated weaponry.
In 2008, things started to sour.
During clashes with supporters of key Sunni politician Saad Hariri, Hezbollah fighters seized control of a section of western Beirut, sparking fears of a new civil war.
The party, and its Syrian allies, were already suspected of having been behind the 2005 assassination of Hariri's father, Rafiq, and four of its members were eventually indicted by a UN-backed tribunal.
Its leader Hassan Nasrallah threatened in 2010 to "cut off the hand" of anyone who attempted to arrest them, and they remain at large.
Prior to Monday's EU decision, Hezbollah had already been blacklisted as a terror group by the United States, Britain, the Netherlands and Australia.
The latest move comes after Hezbollah was implicated in an attack against Israeli tourists in Bulgaria last year and its activities in Cyprus.
Hezbollah's growing involvement in the Syrian conflict in recent months has further worried EU nations.
The charismatic Nasrallah took over as secretary general in 1992 after his predecessor, Abbas Mussawi, was assassinated in an Israeli missile strike.
He acknowledged for the first time on April 30 that his troops were involved in fighting in the Syria, where they helped Assad's troops to recapture the former rebel bastion of Qusayr.
Nasrallah has also warned that the "friends of Syria," a reference to Hezbollah and to Iran, will not allow the fall of Assad, and has promised "victory" in Syria.
Lebanese University professor Waddah Sharara, an expert on Hezbollah, said its militia has about 20,000 fighters, including 5,000 trained by Iran in urban warfare.