FACTBOX: Risks to stability in Lebanon

(Reuters) - Lebanon, which held a parliamentary election on Sunday, has been jolted in the past four years by political killings, a war with Israel, a militant Islamist revolt and the worst internal fighting since the 1975-90 civil war.

Following is a review of threats to stability:

INTERNAL POLITICAL AND SECTARIAN TENSIONS

The security threat posed by rivalry among Lebanese politicians has diminished substantially since May 2008, when their power struggle spilled into violence in Beirut and other parts of the country. A Qatari-mediated deal that produced a "national unity" government has contained tensions, helped by a diplomatic thaw between Saudi Arabia and Syria, whose support for rival factions had made the crisis more volatile. It has been eight months since a Lebanese politician was assassinated.

However, the conflict has left deep sectarian divisions between followers of the rival leaders. Communal tensions could quickly generate more violence were the domestic political climate to deteriorate, or the regional detente to collapse.

An international tribunal to try suspects in the 2005 assassination of former Prime Minister Rafik al-Hariri also remains a potential trigger for instability, depending on any indictments issued by the prosecutor.

CONFLICT WITH ISRAEL

Israel and Hezbollah, an Iranian-backed Lebanese military and political group, have not exchanged fire since a 34-day war in 2006 that was halted by a U.N. Security Council resolution.

Both sides sought to avoid another conflict during Israel's Gaza offensive in December and January. Hezbollah denied any role in rocket salvoes fired into Israel from south Lebanon. Israel responded with artillery fire on unpopulated areas.

Hezbollah deputy leader Sheikh Naim Kassem told Reuters on March 9 he saw little prospect of another war with Israel soon. But any Israeli attack on Iran over its nuclear program could spark another conflict between Israel and Hezbollah, which shares Iran's Shi'ite Islamist ideology and was founded by the Iranian Revolutionary Guards after the 1982 Israeli invasion of Lebanon. Hezbollah has expanded its arsenal since the 2006 war, according to Israeli officials and the group's own statements.

MILITANT ISLAMISTS

Lebanese security forces have sought to crack down on al Qaeda-inspired Islamists who have tried to build a presence in Lebanon in the last three years. In 2007, the army put down an insurrection by the al Qaeda-inspired Fatah al-Islam group in a Palestinian refugee camp in the north. The 15-week battle killed at least 430 people. Sunni militants remain in the southern Ain al-Hilweh Palestinian camp, which is off limits to the security forces. But al Qaeda followers have been unable to build an organization as potent as Fatah al-Islam was before its defeat.

PALESTINIAN RIVALRIES

The assassination in March of a senior official in the Palestinian Fatah faction increased tension in Lebanon's already volatile refugee camps. Kamal Medhat's killing was widely seen as linked to an internal Palestinian power struggle. In an April report on security in Lebanon, the U.N. secretary-general cited deepening divisions among Palestinian factions, including those between the Palestine Liberation Organisation and Hamas, as "an additional factor of instability." Lebanon is home to 12 Palestinian camps housing around 200,000 registered refugees.

PALESTINIAN BASES

Armed Palestinian groups were accused of firing rockets from Lebanon into Israel during its Gaza offensive, highlighting a risk to stability from such factions. Outside the refugee camps, Syrian-backed Palestinian groups operate four bases along the Lebanese-Syrian border and another south of Beirut.

(Writing by Tom Perry; Editing by Alistair Lyon)