An interim cease-fire brokered by the U.S. in 1981 among Syria, the PLO, and Israel was respected for almost a year. Several incidents, including PLO rocket attacks on northern Israel, as well as an assassination attempt on the Israeli Ambassador to the United Kingdom, led to the June 6, 1982 Israeli ground attack into Lebanon to remove PLO forces. Operation "Peace for Galilee" aimed at establishing a deeper security zone and pushing Syrian troops out of Lebanon, with a view toward paving the way for an Israeli-Lebanese peace agreement. With these aims in mind, Israeli forces drove 25 miles into Lebanon, moving into East Beirut with the support of Maronite Christian leaders and militia.
In August 1982, U.S. mediation resulted in the evacuation of Syrian troops and PLO fighters from Beirut. The agreement also provided for the deployment of a multinational force composed of U.S. Marines along with French and Italian units. A new President, Bashir Gemayel, was elected with acknowledged Israeli backing. On September 14, however, he was assassinated. The next day, Israeli troops occupied West Beirut to secure Muslim militia strongholds and orchestrated the massacre of almost 800 Palestinian civilians in the Sabra and Shatila refugee camps, in operations "Spark" and "Iran Iron Brain" within IDF's invasion plans of Lebanon. Israel's then-Minister of Defense Ariel Sharon was held directly responsible for the massacre by the Kahane Commission and later resigned. With U.S. backing, Amin Gemayel, chosen by the Lebanese parliament to succeed his brother as President, focused anew on securing the withdrawal of Israeli and Syrian forces. The multinational force returned.
On May 17, 1983, Lebanon, Israel, and the United States signed an agreement on Israeli withdrawal that was conditioned on the departure of Syrian troops. Syria opposed the agreement and declined to discuss the withdrawal of its troops, effectively stalemating further progress. In August 1983, Israel withdrew from the Shuf (southeast of Beirut), thus removing the buffer between the Druze and the Christian militias and triggering another round of brutal fighting. By September, the Druze had gained control over most of the Shuf, and Israeli forces had pulled out from all but the southern security zone, where they remained until May 2000. The virtual collapse of the Lebanese Army in February 1984, following the defection of many Muslim and Druze units to militias, was a major blow to the government. With the U.S. Marines looking ready to withdraw, Syria and Muslim groups stepped up pressure on Gemayal. On March 5, 1984 the Lebanese Government canceled the May 17 agreement; the Marines departed a few weeks later.