What was the Litani River Operation?

Using Lebanon as a base, Palestinian fighters mounted intermittent cross-border attacks against civilian and military targets in Israel. There were also international terrorist spectaculars, e.g., the 1972 Munich Olympics massacre, perpetrated by groups based in Lebanon. In turn, the Israelis struck back at targets and groups across the border in Lebanon from time to time.

After the March 11, 1978 Bus Hijacking on the Coastal Road, Israel launched a major military incursion into South Lebanon, called the Litani River Operation. Israel crossed into southern Lebanon on March 15 and struck at PLO terrorist bases and staging areas south of the Litani River, up to ten kilometers deep inside the country. Twenty-one IDF soldiers were killed before the operation ended on March 21, 1978.

The operation prompted a formal statement of "United States Concern With the Territorial Integrity of Lebanon," calling for Israeli withdrawal and discussing a United Nations role in Lebanon. On March 19, 1978, the United Nations Security Council adopted Resolution 425 calling for Israeli withdrawal and establishing an international peace-keeping force for South Lebanon, the United Nations Interim Force in Lebanon (UNIFIL), to enable the establishment of a buffer zone in southern Lebanon free of terrorists.

By June Prime Minister Begin, under intense American pressure, withdrew Israeli forces, which were replaced by UNIFIL. The withdrawal of Israeli troops without having removed the PLO from its bases in southern Lebanon became a major embarrassment to the Begin government. The cross-border cycle of attack and retaliation continued, and the PLO expanded its bases, forces, and armaments in Lebanon. Finally, Israel was forced to respond with its 1982 invasion and occupation of southern Lebanon. The UNIFIL force of about 3,500 is still deployed in 2002.

Sources and additional reading on this topic:

Lebanon
UNIFIL
The history of the PLO and the International Legitimization of Terrorism
Israeli Action in Lebanon, 1978-82
UN Security Council Resolution 425
Why Israel rejects "observers"