Some pertinent Lebanese history, courtesy Knowledge News:

...By the late 1940s, [Lebanon's] makeup was changing, and many Muslims complained that Christians--about half the population--enjoyed an unfair share of wealth and power. Tensions increased when Palestinian refugees poured into Beirut in the wake of the First Arab-Israeli War in 1948-49. Before long, a brief civil war broke out, but it settled nothing. Then, in 1967, another Arab-Israeli war, the Six-Day War, drove another wave of Palestinians into Lebanon. With them came the heavily armed militias of the Palestine Liberation Organization (PLO), which started to use southern Lebanon as a base for operations against Israel. Dispute over what to do with the Palestinians helped polarize Lebanon, exacerbating the old ethnic and religious tensions. While Muslim and leftist leaders called for a new census and a new political structure, Christian leaders decried the PLO's expanding power and influence. Armed militias cropped up on both sides. Then, in 1975, a full-scale civil war erupted, with Beirut as the primary battleground. In the chaotic 15 years that followed, Beirut was divided into two--with Christians in the east and Muslims in the west. Much of the city was reduced to rubble, and many of the region's powers were drawn into the fray. Syrian forces first arrived in 1976, seeking to prop up the existing government. During the relative calm they helped impose, the PLO launched a series of attacks against Israel, which responded by invading Lebanon in 1978. The Israelis soon withdrew, but invaded again in 1982, determined to destroy the PLO. That invasion led to the infamous Sabra and Shatila massacres, in which the Israelis allowed Lebanese Christian militiamen to massacre hundreds (perhaps thousands) of Palestinian refugees in two camps. It also led to the creation of a multinational force--including U.S. Marines--charged with stabilizing the city. In October 1983, truck-bombing terrorists killed 241 U.S. soldiers and 58 French soldiers who were serving as part of the multinational force. The United States blamed the attack on the Shi'ite militant group Hezbollah and its Iranian backers. Early the next year, the multinational force went home, and the war went on. In 1986, leaders in West Beirut invited the Syrians to send enough troops to stop the fighting. The Syrians complied, but peace didn't come until 1990--and when it did come, the Syrians didn't bother to leave. As the Beirutis began to rebuild their battered city, the Syrians effectively assumed control of Lebanese politics. In September 2004, the UN Security Council told Syria to send its troops home. But the Syrians didn't budge until the so-called "Cedar Revolution" of 2005, when huge protests within Lebanon and international pressure pushed them out. By then, most of the old militias had disbanded. But not Hezbollah, which had replaced the PLO as Israel's main antagonist in southern Lebanon. There, the conflict has never really ceased.