After the 1948 Arab-Israeli war, Lebanon became home to more than 110,000 Palestinian refugees, after their settlements in Palestine and Israel had been depopulated as a result of the war. The large influx of Palestinians from Jordan after "Black September" caused an additional demographic imbalance within Lebanon. By 1975, the refugees numbered more than 300,000 and the PLO in effect created an unofficial state-within-a-state, particularly in Southern Lebanon. These rapid demographic changes eventually spark the Lebanese Civil war and continual and escalating violence near the Israeli-Lebanese border occurred between Israel and the PLO .
In July 1981 the US negotiated a ceasefire between Israel and the PLO. The ceasefire was violated one year later, on June 3, 1982, when a member associated with the Abu Nidal Organization attempted an assassination against Israel's ambassador to the United Kingdom, Shlomo Argov. This attempt on the ambassador’s life was being used as justification for the invasion, but finally gave Israel an opportunity to do what they had been trying to do. IDF forces marched into Lebanon on June 6, 1982 as the government announced that Israel’s objective was to push the armed groups northward, thus ensuring the northern Israeli communities were safely out of fire range. The operation was meant to be no more than 48 hours long, reaching 25 miles into Lebanon. IDF forces, however, ended up reaching the outskirts of Beirut.
By June 14, IDF forces were surrounding Beirut. Their plan was to impose a “new order” in Lebanon. Israel was to help Lebanon become free of Syrian and Palestinian influences, thus securing peace for both sides. Christian forces leader Bashir Gemayel was soon elected president of Lebanon, but just several weeks later he was assassinated in Beirut. Shortly after the news of the assassination broke, IDF and Christian forces entered western Beirut. The Christian Phalangists who entered the Sabra and Shatila