Saudi Arabia's King Abdullah will visit Syria on Thursday to discuss ways of calming tension in Lebanon over a United Nations tribunal to try suspects in the 2005 killing of former Lebanese premier Rafik Hariri.

Here are questions and answers on why Syria retains influence and interest in neighbouring Lebanon despite having withdrawn thousands of troops following Hariri's assassination, which many anti-Syrian politicians blamed on Damascus.

What is the nature of ties between Syria and Lebanon?

  • Since it was forced to end its nearly three-decade long military presence in Lebanon, Syria has been gradually rebuilding relations and restoring its influence.
  • Shortly after becoming prime minister, Sa’ad Hariri -- son of the assassinated billionaire statesman -- visited Syrian President Bashar Al Assad in December 2009, turning the page on nearly five years of animosity between the two leaders.
  • Since then, Hariri has been to Damascus three more times and Lebanese President Michel Suleiman has also visited.

What happened to the anti-Syrian alliance?

  • Hariri has toned down his anti-Syrian rhetoric and his government, which includes pro-Syrian and Hezbollah ministers, has pledged to have good relations with Damascus.
  • A once staunch critic of Syria, Druze leader Walid Junblatt broke away from the anti-Syrian coalition known as "March 14" earlier this year. Junblatt had described Assad as a "monkey, snake and a butcher" in 2007, but later retracted the comments and visited Syria after mediation by Hezbollah.
  •  Syria's rapprochement with regional Sunni Muslim power Saudi Arabia, which backs Hariri, has echoed positively in Lebanon. The thaw in ties between the two countries -- which back rival political players in Lebanon -- helped Hariri form a national unity government after defeating the pro-Syrian Shi'ite Hezbollah movement and its allies in a 2009 election.
  • Last year Abdullah made his first visit to Damascus since the 2005 assassination.
  • While Hariri used to repeatedly say that an international tribunal investigating the murder of his father would bring the Syrians to justice, he has changed his stance to say he would accept whatever findings the tribunal comes out with.

What is the impact of the Hariri investigation?

  • The United Nations investigation initially implicated high-level Lebanese and Syrian security officers in Hariri's killing. Damascus denied involvement.
  • Four pro-Syrian Lebanese generals were detained for four years without charge in connection with Hariri's killing. They were released last year after the court said there was not enough evidence to indict them.
  • After years of focus on Syria's role, attention has turned to a possible link with Hezbollah. The group's leader, Sayyed Hassan Nasrallah, said this month he was told by Hariri that the tribunal would indict Hezbollah members, not Syrian officials.
  • Nasrallah's condemnation of the investigation as an Israeli project escalated tensions in Lebanon and any indictment of a Hezbollah member could destabilise the unity government.
  • Trying to avoid a repeat of the divisions that led the country to the brink of renewed civil war in 2008, Hariri and Suleiman have sought to calm the situation.

How have diplomatic, trade relations developed?

  • Syria and Lebanon opened embassies and appointed ambassadors in the last two years, the first time they have had diplomatic relations since Britain and France carved them out of the old Ottoman Empire in 1920.
  • The one-year anniversary of the Syrian ambassador in Lebanon was marked by a lavish ceremony.
  • During Hariri's last visit to Damascus, the two sides signed economic cooperation accords, leaving unresolved border demarcation, which Beirut sees as central to its sovereignty.
  • Trade between Syria and Lebanon slowed in recent years, partly due to political tensions, but Lebanon remains a banking centre for Syria and many Syrian labourers work in Lebanon.

Why does Syria back Hezbollah?

  • Syria's last round of Turkish-mediated peace talks with Israel collapsed in 2008. In the absence of peace, Damascus is able to project regional power through its influence in Lebanon.
  • Syria consistently lost on the battlefield against Israel, but has relied on Hezbollah forces to thwart Israel's ambitions in Lebanon and fight a proxy conflict with the Jewish state.
  • Syria has been accused of re-arming Hezbollah since the 2006 Israel-Hezbollah war, in which the Shiite guerrillas held their ground.