The Shiite militant group Hezbollah, apparently the destination for the arms shipment targeted by two Israeli air raids in 48 hours inside Syria, is a staunch ally of Iran, that oversaw its creation in 1982, and the Syrian regime, one of its key military backers.
Hezbollah, or Party of God, was founded in 1982 in response to Israel's invasion of Lebanon, and has since gained enormous support among Shiites, thanks in part to its extensive social services network.
The movement, which drew inspiration from the Iranian Revolution, receives financial and military backing from Tehran.
It is also supported by the Syrian regime of President Bashar al-Assad, which has been accused of smuggling sophisticated weapons to the party through the porous Lebanese-Syrian border.
Hezbollah's main objective since its creation has been its armed struggle against Israel.
Its popularity rose in 2000 after the Jewish state withdrew its troops from southern Lebanon following a 22-year presence.
The party's political clout was also bolstered after its devastating 2006 war with Israel, and again in 2008 following a wave of sectarian violence that saw it stage a spectacular takeover of mainly-Sunni west Beirut.
But the war in Syria that broke out after the regime violently repressed a popular uprising in March 2011, has dented Hezbollah's image in the Arab world, as the movement has taken an unambiguous stance in favour of the regime.
The group's leader, charismatic cleric Hassan Nasrallah, admitted for the first time on April 30 that his troops have been involved in fighting in the Syrian border region of Qusayr.
He warned in a speech that the "friends of Syria" will not allow the fall of President Bashar al-Assad.
Syria's opposition immediately denounced the statement as a "threat."
Last March, US President Barack Obama called on the international community to name Hezbollah as a "terrorist organisation."
It is the only group in Lebanon that has refused to surrender its weapons since the end of the 1975-1990 civil war on grounds they are needed to protect the country against Israeli attacks.
Nasrallah took over as secretary general of the party in 1992 after his predecessor, Abbas Mussawi, was assassinated in an Israeli missile strike.
He has since become one of the most influential leaders in the Arab world, has lived in hiding since the 2006 war and rarely appears in public for fear of being killed by Israel.
In November 2010, Nasrallah threatened to "cut off the hand" of anyone who attempts to arrest Hezbollah members over the 2005 assassination of former Lebanese prime minister Rafiq Hariri amid press reports at the time suggesting that senior Hezbollah members would be indicted in connection with the murder.
The party, or some of its affiliates, have been implicated in a number of attacks, notably the bombing of the US marine barracks in Beirut in 1983 that left 241 dead and the 1992 attack on the Israeli embassy in Argentina.
Intelligence officials estimate Hezbollah has amassed an arsenal of more than 40,000 short and long-range rockets as well as other sophisticated weapons including anti-aircraft guns the party claims can reach deep inside Israel.
Although Hezbollah initially sought to impose an Islamic way of life in Lebanon, it has since toned down its rhetoric and claims to seek to live in unity with all the Lebanese.
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