A war between Israel on the one hand, and the PLO and Syria on the other, which broke out on 5 June 1982. The war began as a limited Israeli operation (Operation Peace for Galilee) against PLO strongholds in southern Lebanon, following sustained attacks against civilians in the north of Israel, as well as the attempted assassination of the Israeli ambassador to the United Kingdom. Israeli operations were extended as it became embroiled in the ongoing Lebanese Civil War, and Israel eventually occupied large parts of Lebanon, including western Beirut. In what became known as the Sabra and Shatila Massacre, Christian militias attacked two Palestinian refugee camps in Beirut, killing hundreds to thousands of civilians. Under intense Israeli pressure, the PLO leadership relocated from Beirut to Tunis in 1982. In 1983, a non-belligerency treaty was signed between Israel and the Christian-backed Lebanese government, but it was cancelled the following year. By 1985, Israel gradually withdrew its forces from most of Lebanon, forming the South Lebanon Security Zone. Low-intensity warfare between Israel, the Israeli-allied militia South Lebanon Army and Hezbollah continued until Israel's complete withdrawal from Lebanon in 2000, thereby implementing UNSC Resolution 425 of 1978.